Could be...
He's doing Deca or Tren or something else that makes his hard-on's less hard and full, so he's smaller. He should put some Hot Stuff on there before sex - that'll do the trick... LOL
Could be that she doesn't like him juicing and is screwing with his ego.
Other (terrible) scenario is that she's cheating on him with some guy who's hung like a brontosaurus and wants to have an excuse as to why she doesn't feel the same to him.
Or, maybe his dick IS shrinking! I talked to a guy who knew a guy who trained with a guy who was a lot bigger than me and he was juicing for like 3 months straight and his dick shrivelled up, got all black and he had to stop b/c he thought it was going to fall off... But that's just what I heard.
-R