As poker players we try to follow betting patterns to figure out what an opponent may be holding. We mentally create a range of possible hands they could be on, and then eliminate hand possibilities based on the betting patterns that transpire during the hand. But many players fail to incorporate the bet size into determining whether an opponent has the goods or not. This information can sometimes help determine if they have the goods or not, which can make all the difference in stack size.
If an opponent has been betting a smaller percentage of the pot on the flop and turn, only to bet an overly large amount on the river, it’s likely that this player has missed a draw or hit two pair on the river. The smaller bets on the earlier streets could possibly be semi-bluffs from a player on the draw. The only other situation that would produce a larger bet size on the river would be two pair. You will be able to see from the board cards if there is a flush or straight draw on the flop that never got there by the river. If there is, it could be the missed draw. If there isn’t, they likely have two-pair.
If a player has been making larger bets on the earlier streets, only to reduce the percentage of the bet on the river, it points to two possible things. Either they have a good hand and are betting a smaller amount so they can get a little more value on their hand, or they think you may have been on the draw and they are taking one last stab at the pot with their weak hand.