I just realized something seemingly silly I do all the time. I raise to 3x the bb preflop and one person calls. I hit a pair on the flop and bet ~20% of the pot. (There is no flush draw on the board in this case, otherwise I bet big enough to price the draw out). It just occured to me that I am betting less postflop than preflop, which doesn't really make sense since the pot is larger and I may look weak.

So, and hopefully this isn't too general a question, how big should post-flop bets be? What would be the minimum? 30% of the pot? 40? 50?