My green brain has very strange patterns too... Some days it'll will stay shrunk up and others it will be fully extended ... It worried me at first but as long as I don't see any wounds I don't worry about it now. I don't feed it all
My acans do the same thing, through out the day they are puffed up when all 8 lights are on, when 6 turn off they deflate. My guess would be its your lighting.
Strange..... My rainbow brain is huge during the day and at nights it takes several hours to "deflate" always looks cool though... You could try something like polyp booster from poly labs to see if it's puffed up when feeding. I used some tonight and my brain looked like it could explode!!!! Lol that stuff triggers some insane polyp extension and feeding response in everything in the tank....