If there are limpers, it decreases your chances of having your premium hand hold up post flop. If I'm in a game where only crazy bets will make that guy with the suited 4 gap connectors fold, I feel that I need to raise at least 7 or 8x the BB to at least make him consider folding, where if I'm in a game with tighter players if you raise that large holding AA it will even make the guy holding JJ fold out and you want to keep him in the hand.

tha'ts why I vary my raise sizes from table to table, but keep it pretty consistent while at a table.