Why do corals shrink with more flow?!

[frizzayyyyreef]

I only have one power head and never usually even use it I just have the one pump on the top I forgot the name it’s not a power head tho...so when I do put mine on a lot of my lps corals seem to get smaller...but if I turn off ALL the flow they get hugeeee and look crazy healthy I know flow is important so I was jw why this happens

 

[frizzayyyyreef]

Also when my power head is on its indirect flow it’s not pounding any corals it’s actually aimed at the back of the tank hitting the wall fyi