Quote from saxon22:
I wonder how some of the successful traders move from that proverbial 1 contract to trading 10 to 50 or even 100?
By the same token, those of you who are successful in trading futures, why aren't you increasing your size to 10, 50 or 100 contracts?
Quote from Ripley:
It has to do with the Theory of Constraints...
You can only handle upto what your weakest link can handle. Oftentimes, the weakest link is the trader's own perceptions.
Next up is the Marginal utility of the next dollar is very slim for most successful traders. The trouble, risk or trading one more contract isn't worth the potential $ that it can make.
Next up is the Liquidity issue, I know it shouldn't be a big issue on huge markets like the S&P, but not all traders like to trade the S&P. It is also easier to catch quick moves with huge gains with a little smaller size.

Quote from 2006:
Size depends a lot on your account size.
Best way is to gradually ramp up (as long as your performance is positive and consistent).
Quote from ess1096:
Marty Schwartz gave this advice in the book "Market Wizards";
"Donât increase your position size until you have doubled or tripled your capital. Most people make the mistake of increasing their bets as soon as they start making money. That is a quick way to get wiped out.â