Concept: Gods that become less powerful the more people believe in them.
Necoho of Warhammer FB fame works like this, except he’s the god of disbelief in general
His existence is kind of a paradox and so he tries to maintain as small a cult as possible while still maintaining some kind of presence amongst mortals
It could be as simple as when number of believers increases, the disagreement between them also does. Denominations, sects, schisms, etc. They’re all the “same” religion but they all diverge a little or a lot. So, as the believers grow, so does who the god is–and thus the god becomes less powerful because it’s all different versions, all different pieces vying with each other. The whole “house divided cannot stand”, and such.
Or you can have a unified cult that’s worshipping an idea of a god that isn’t even really close to what the actual god is like, thus meaning all of their power does nothing for his strength, see: Small Gods (albeit there Pratchett was mostly flip about ossification of Christianity)