Difficulty has doubled (almost i think) so any growth in the pool would have to take that into consideration... Further, as pool hashing increases in magnitude, unless your hashing personally has increased to match, you represent a smaller part of the solution then what you once did. This combined with the increased difficulty means that your contribution in shares is less as a proportion of the total shares req'd for solution; therefore lower payouts.
So long story short, if the pool doubles in power, and difficulty also doubles, then you will get less payout then what you did before on average. Welcome to the difficulty conundrum, and why so many are getting out of mining period. The difficulty is not likely to ever get easier from here till the end.
I understand fully that I should be getting paid less when the pool increases in size, but we should also be discovering blocks faster. If you look at the stats and add everything up, we solved the same number of blocks on the first full day of the last difficulty as the last full day even though the pool grew from 350ghash to over 500ghash. There was variance inbetween of course, but not the growth in found blocks one would expect to see from a 43% gain in hashing power. Idk maybe I am wrong.
But a 43% gain in hashing power versus what growth in difficulty? The way I see it without breaking out a calculator, is that the difficulty has increased by at least 43% if not more... Wasn't it like 5xxxxx vs now 8xxxxx? Which would be something akin to a 60% increase in difficulty (pulling it right out of my axx, as I don't know if 'difficulty' as reported is a linear progression or not - but the numbers seem to indicate it is)
So with a 60% increase in difficulty on a sufficiently large sample of blocks, the 43% increase in hashing power wouldn't cover the increase in difficulty fully. So that means blocks solved on average goes down per unit of time (averaged over a given amount of time that is sufficient to show a proper distribution) and our payouts based on a smaller proportion of shares and a smaller number of solved blocks faces a double whammy deflator.
If however difficulty growth = hashing growth, then we would solve the same number of blocks as a pool, and yet still receive smaller payouts unless our personal hashing increased to match the pool.
Sadly, with the amount of horsepower that came online after the BTC spike, and the media coverage... Expect that the next difficulty increase may be even higher then this one. The system is designed to balance the amount of blocks generated to a certain rate, accounting for increased processing power.... Which means conversely, that if a bunch of people decide instead that because BTCs have dropped, and Mt Gox had its blow up, etc that they will return their hardware to new egg, sell it off, etc... That it is possible (albeit unlikely) for difficulty to go down if there isn't sufficient hashing power to meet the desired rate of solved blocks.
However, the trend will be that difficulty increases every few weeks, and thus our shares will be more and more expensive in terms of heat / electricity / etc.