Muscles looking bigger or smaller from one week to the next

Hope this makes sense, butsometimes my shoulders and pecs look quite developed in the mirror. Not just after a workout but often after a few days of going to the gym
Then a few days later they seem to have shrunk again.

Is there something to that, are there certain factors which make this happen like being dehydrated for example or is it all in my mind?? I'd like to know so I can maximise the muscle development