That is incorrect.

If the overall rate grows you are still compensated the same.

Easy example;

Day 1 - your the only miner in the pool with 1TH, you earn say 0.3 BTC per day and you own 100% of the hash rate.

Day 2 - now there are 4TH total, meaning you own 25% of the overall hash rate, BUT your 1TH still makes you the same 0.3 BTC per day

The only way this changes is when difficult jumps, as you would then earn less per day...

Cheers,