So lighting,flow,nutrients and trace elements all contribute to zoa head size and growth.
I had a problem with keeping nutrients in my pico (maybe because I used live rock,MB7 and pods to seed the tank),but no matter what I did nitrates and phosphates would never stick around.
So I dosed NeoPhos,NeoNitro and my dull z/p's colored up to how they should be,started to grow super fast and their polyp sizes went from the size of a pencil eraser to a dime within a few weeks.
Iodide REALLY helps z/p's but you need to be SUPER CAREFUL when dosing this element and any rule of thumb is whatever you dose (trace element wise) you need to test for.