Definition of pot size bet
A pot size bet is a bet of the size of the pot. If you are on the flop and the pot is 25 and your opponent bets 25, they are said to be making a pot size bet. This is not the norm in poker games. You will obviously see players making pot size bets, but the majority of the time, players will bet 25% pot, 33% pot, 50% pot and 75% pot. These are mostly shortened bets when you play poker at your online poker room.
Why is the pot size bet rarely the optimal bet?
When playing poker, we want our opponents to make mistakes. To get them to make mistakes, we want to give them tough decisions. If a player has a flush draw and I make a pot size bet, I completely discourage him from running his draw. I give him an easy decision and encourage him to fold. By betting less, I still make money in the long run if he has a flush draw and I keep him in the hand. And if I bluff in pot size bet, I lose a lot when I get called. Often a small bet would have done the same job.