[Kurn]

In a tournament, you generally raise to either limit the field or steal the blinds. The concept that I learned from Greg Raymer was that the basic idea was to make the smallest possible raise that accomplishes those goals: maximize fold equity or when the blinds don't fold, get it HU with position. As TP said, this comes into play later in a tourney when the blinds/antes are high relative to stack size and you have a big stack.

In a cash game, winning the blinds tends to be irrelevant, so you add building a bigger pot as another reason to raise. Smaller raises serve that purpose, but add the stress that you better be sure you can get away from the hand postflop when you're in trouble. Since I'm not a NL cash-game player, that's all I'll say about that aspect.

As for varying preflop raise sizes, although the conventional wisdom on poker boards says always raise the same amount, Harrington, Sklansky, Lindgren, Negreanu and Ferguson all disagree. Harrington says randomly vary preflop raises between 2x-5x, Lindgren prefers to minraise, and Ferguson says there's little reason to raise more than 2x from EP. personally, some of the opponents I've found to be the toughest in tourneys are those who play a lot of hands and always minraise.

One final thought is that the "always raise the same amount" justification is to prevent your opponents from being able to figure out your hand by the size of your raise. However, we can infer that the above named players don't necessarily think that's much of an issue, and they play in games where one would suspect that their opponents actually try to figure out what their bets mean. On the other hand, at the levels I play, the under $30 online tourneys, its probably safe to assume that the average opponent isn't really thinking about much more than his own cards.