If you have a read through this summary or watch the video, Liz explains how nash equilibrium is achieved for staking pools (graphs are included): Summary of Project Shelley (June 2018) Update
Otherwise, this summary post on incentives and delegation goes a little more in depth into the basic idea of how rewards are distributed which helps achieve pools of equal size. Scroll to section “Basic idea of distribution: Explained in Two Steps” : Summary: PoS Delegation & Incentives (Lars Brunjes)