i.e. If you have 100 rigs setup with 1 GPU you have 100 lottery tickets to be the first to get a LP as opposed to 20 rigs with 5 GPU's where you have only 20 tickets....
For the really short rounds (in the hundreds/few thousands of shares) your hashing power isn't so much what counts as it is the time in which you get the LP (and then of course deliver a solution before the round ends)
(yes that is a rather controversial question, but only if the answer isn't truly random
So that effectively means that the higher your hash rate the more chance you have of submitting a share in a very short round.... so smaller miners will have less of a chance to participate in a 'more profitable' round than big miners. That means quick rounds don't even out based on variance... so the model becomes statistically biased as big miners will have a bigger chance to begin with (of course you have to factor in geographics / network speed / etc etc)
Edit: Mind you, it is all mostly a hypothetical discussion as there aren't that many rounds where not all miners participate (on Deepbit) and even though it would be nice to get a 'huge' payoff in a round from time to time, I have the feeling that not participating in such a round won't affect your overall 'expected' payout that much....