I'm surprised you're oblivious to this. I thought it was just common knowledge at this point.
The same thing happened with Bale while filming BB, I had heard that Bale kept getting bigger and bigger causing the suit to fit him tighter to which the costume designer had to constantly "adjust" the suit, mind you this was a rubber suit a much harder material than Brandon's Superman suit, while Bale's Bat suit would get tighter Brandon's Superman suit would tear.
I could be wrong about Bale gaining muscle maybe I heard wrong and in fact he loss muscle either way the BB suit was constantly "adjusted".