“Why so much?” It's one of the most common questions you'll hear at a poker table (in a big bet game, anyway). But like so much of the information gathering you see, this line of thinking seems to lead the questioner astray at least as often as it leads her towards the best play.
The idea seems to be that a big bet requires explanation, that some bets are “normal” or “standard” and therefore mean nothing, and that others are uncommonly large or small and so must contain information about an opponent's hand.
It's telling that players disagree about what these deviations mean, though. Some people tend to assume that large bets are generally bluffs; I've even seen large bets contrasted with a “value bet” in a way that implied a value bet would necessarily be smaller. Others will assume that big bets always mean big hands.
To be fair, there is some truth to the idea that certain bet sizes reveal information. Many players really do restrict themselves to betting within some narrow band of fractions of the pot, often 50-80% or so, and deviate from these sizes only for very specific reasons. If you can determine those reasons, then you can really determine the strength of their hands with a high degree of accuracy.
However, there are many situations where very large (or small, for that matter) bets don't have to mean anything in particular. They can easily be made with a balanced range, in a way that does not cost the bettor anything with any individual hand in that range relative to a different bet size he could also employ. Learning to distinguish between situations where large bets can be balanced and situations where they can not is the key to determining whether your opponent is revealing anything with any particular bet.
The most important thing to recognize is that larger bets generally require more polarized ranges. Intuitively, this makes sense. If I make a bet that is small relative to the pot, the odds will compel you to call even with relatively weak hands, which means that I don't need a monster to be ahead of your calling range. If I bet more than the pot, then you won't call so often, and when you do call, you'll have a stronger hand. Consequently, I can make that bet either for value with very strong hands, or as a bluff with weak hands, but not for thin value with medium-strength hands.
The major point of confusion here is when people assume that polarized ranges are necessarily weak. It's a lot easier to make a weak hand than a strong one, the logic goes, so if you can narrow me down to one or the other, shouldn't you expect me to have the former a lot more often than the latter?
Not necessarily, for two reasons. The first is that, even if I have more weak hands available to me, I am allowed to check as many of them as I like. I can get to the river with 80% air and 3% monsters and still make a balanced overbet by checking the vast majority of the air.
It's not hard to construct a scenario where you should strictly prefer folding bluff-catchers to such a bet. Suppose an obviously recreational player has barely entered a pot for several hours. After checking his option in the big blind and then checking twice on a K775 board, he suddenly moves all-in for several times the pot on a 2 river. He's never had a chance to fold his air, so he's got plenty of it with which he could theoretically balance this bet (or even be unbalanced towards bluffs), but I would nevertheless fold 76 without hesitation.
The second problem with this logic is that it is not always so easy to have weak hands. Suppose a player has called large bets on an A962 board. The river is the T, and now he overbets the pot. Even if you conclude that he's either bluffing or value betting a very strong hand, you can comfortably fold a set. By calling large bets on the flop or turn, this player is telling you he has either a hand with significant showdown value or a draw. On the river, basically all of the draws get there, and his other holdings probably have too much showdown value to turn into a bluff, so he will have a straight or flush far too often for you to call with a pure bluff-catcher.
Players who can't bring themselves to fold a set will often tell themselves that their opponent could have AK or A9, so it's worth going into a bit more detail about why you shouldn't expect to see those after such a large bet. They are, after all, strong hands, quite possibly strong enough to bet for value in many situations. Here, however, your opponent has so many stronger hands and so few bluffs in his range that you have no incentive to call an overbet with hands that would lose to A9. After all, we're talking about folding better.
That brings us around to another potential objection: could your opponent be turning AK into a bluff? It's possible, but it's unlikely that this is a profitable move for him, and if it is, there are probably better ways for you to take advantage of that.
In order to make your opponent indifferent to bluffing, you need to call or raise often enough that the EV of his bluff is the same as the EV of checking. When your opponent holds a hand without showdown value, then the EV of checking is 0, and you must call with a frequency equal to the size of the bet divided by the size of the pot so that the EV of bluffing is also 0.
However, if the EV of checking is greater than 0, which it probably is in this case, then you should not call that often. If you call at a frequency that makes bluffing with AK have an EV of 0, then you are getting exploited by an opponent who checks, because that will have a higher EV for him than bluffing.
But let's say that your opponent makes a mistake and bluffs with AK even though checking might be better against your range. If you knew that he would have AK in his betting range, does that mean you should call with your set?
Again, not necessarily. The mere possibility of a bluff is not enough to warrant a bluff-catch. You must consider whether there are enough bluffs to compensate you for the many strong hands he could hold.
If your opponent really is turning so many strong hands into bluffs after you check the river, then while it may be correct to call with your set, you should consider some other adaptations as well. This player is probably incentivizing you to check all of your strong hands on the river. Rather than betting half the pot when you have a flush and praying for a call from top pair, you'd be better off checking and letting him overbet his top pair into your flush.
Of course, this requires thinking and strategizing about your opponent's likely mistakes before acting on the river. The reason I'm often suspicious of arguments like “Maybe he's turning AK into a bluff” is that they come across as post hoc justifications. It's one thing to say, “If I check this set, he'll overbet with AK and I can call.” It's another to check, face an overbet, and then decide it could be AK. The latter sounds suspiciously like you're looking for an excuse to make what is ultimately a curiosity/frustration call.
The above are examples of cases where it is difficult or impossible to balance an overbet profitably. Either the player does not have enough bluffs available to him, or he has more profitable options with the hands that could potentially be used as bluffs.
In other spots, however, an overbet does not have to read as either strong or weak. Suppose that a button raiser bets the flop and turn into a big blind caller on AK59 and then overbet shoves a 2 river.
In this case, both players' ranges should be wide enough that the button can overbet even a good two-pair for value. This is because, as a button raiser, he also has plenty of bluffs available to him, which means that the big blind can't just trivially fold a hand like AT to a river shove. This is particularly true when the big blind has little or no AA, KK, or AK in his pre-flop calling range.
Of course, against a particular player, you may well be able to call or fold exploitably. But unlike in the previous examples, there is not necessarily an exploit here. You may simply be indifferent or close to indifferent with a big chunk of your range. This is the kind of spot exceptional players know how to make the most of, and like a pre-flop bad beat, there really isn't anything you can do about it except shrug and accept it.
Sometimes, the answer to the question “Why so much?” is simply, “Because this is a good spot to overbet a balanced range.”