If you would calculate it by comparing your contract hashrate to the global hashrate and difficulty you would et a more accurate answer...
So the pool IS irrelevant as the data to accurately predict your earnings is available and outside of the pool..
I'll try to explain it again. The size of a pool has absolutely noting to do with how much you make with your hashrate. Zero. Nada. Nill.
Whatever they tell you, you should get your share out of the total ammount of hashrate, because this is ulimately what drives difficulty and thus is what decides the chance you have of winnig with the hashrate you use.

And again, if total hashrate increases your pool will get less blocks and you will earn less. This is exactly how it should be. You still earn the same ammount per block, but your pool just finds it less often.
If your pool addes a shitload of hashpower you wuld still not earn more because the pool would find more blocks but your hashrate is small compared to your pool and you still get the same ammount less. Just as if that hashrate was added outside of your pool.

To make the story short, in bitcoinland everything adds up (or should).
Bitcoin difficulty directly dictates how much you will earn with a certain ammount of bitcoin.
And difficulty is adjusted because of total hashing power, not just your pool.
Any hashrate on the network is always in direct competition with whatever you have.

Anyway, you'll understand this when the fog clears..