I think most bivvies "might" suffer the same problem Rik.
I got a Fishrite one recently off one of the lads.
He is one of these blokes who is fussy and particular, but really looks after his his gear.
Anyhow he was showing me how he packs his away. Make sure the doorway is zipped up. Each time he folds it over to pack it up, he smoothes the creases out and gets as much air out of the folds as possible. Also on each fold he makes it as square or obolong as possible before he makes the next fold. Otherwise you can end up with odd triangular shaped bits sticking out everywhere. Finally he will tie a piece of cord around it to compress it further if need be. With the Fishrite he uses the tension strap to tie it with.
Alternatively can you buy a bag for a two man?