Quote from kut2k2:
Not really. Position sizing influences whether or not you can continue to exploit an already existing positive expectation.
Consider a situation where you have a biased coin that lets you win 60% of the time. So your expectation is 0.6(+1) + 0.4(-1) = 0.2. Position sizing says to only bet 20% of your betting capital in this situation because that is how you maximize your long-term betting account. You can bet 100% if you want but once you lose a bet, you've lost it all and can't continue. Your expectation is 0.20 per bet in either situation, but only using sensible position sizing will allow you to continue betting and attain a long-term acceptable betting account.
For trading, this means that you must first find an edge (a trading strategy that can expect to win more money than it loses) before you can apply position sizing to take maximum advantage of your edge.
Position sizing will never turn a zero-or-negative-expectation situation into a positive-expectation situation.