The size of a pool, its total hashrate and the distribution of hashrate between bigger and smaller miners, have no effect on the rewards you, mining with a specific hashrate, will obtain on average.
The total block rewards collected by the pool are proportional to the number of blocks it finds per time unit, which is proportional on average to its total hashrate. In every fair pool reward method, the rewards you get on average are exactly proportional to your part in the total work done by the pool. If you mine in a pool twice as large, the pool will collect twice as much rewards but your share in them will be cut by half, meaning you get the same on average.
More specifically, all fair reward methods give miners on average (1-f)pB per share they submit, with f being the fee, p the probability that a share will be a block, and B the block reward. This amount does not depend on anything else, in particular not on the pool size.
What does differ according to the size of the pool is the variance in the rewards (how much they vary from the average due to randomness) and the maturity time (how long it takes to obtain the rewards). It is always the case that a bigger pool will have less variance and maturity time, and thus for a miner of any size it is better to mine for a pool as large as possible (however, it is better yet to mine for multiple pools simultaneously).
One point where a miner needs to optimize based on his own size is the share difficulty; smaller miners will want easier shares, so they should make sure their pool offers it. While bigger miners should work on more difficult shares to make sure the pool server isn't overloaded.
Are you asking because yourself you don't know, or because you want there to be an answer in SE? – Meni Rosenfeld – 2013-06-08T18:18:48.050
I know the answer. But I've been running a pool for 2 years and get these questions and "theories" all the time. I got tired of explaining and wanted an SE page to refer to. Some don't even believe me - maybe they believe it if it is on SE. :) – Dr.Haribo – 2013-06-10T19:58:05.417
1I figured as much, though your wording doesn't really make that clear... Maybe it's better to cut down on the first person. – Meni Rosenfeld – 2013-06-11T06:02:07.480
Yeah, good point. I have now added a short intro text. ;) – Dr.Haribo – 2013-06-11T17:17:22.530
Hm, the body of SE questions shouldn't be too meta either... Maybe the best solution is to clarify with a parenthesized note at the end. Anyway I'll try to whip up an answer sometime. – Meni Rosenfeld – 2013-06-11T17:30:36.393
1I rewrote the question text – Dr.Haribo – 2013-06-16T13:34:20.780