An egg that's noticeably larger than the ones you usually get, is likely to be a double yolker. I recently had a button quail that made one. This was the first egg she laid after being moved (I just purchased her). She laid it about 4 days after her move. In that particular case, I'm thinking the move messed up her egg laying cycle and caused it. But I also think some birds are more likely to lay double yolkers than others - I don't know if she's laid any more since, as the rest of her eggs are relatively well hidden and I don't want to disturb them as I'm hoping she'll go broody.
If it's not a double yolker.. Hmm.. Then it's really a large egg. I think some Tatanka breeders (it's a group of people aiming to breed superior coturnix quail - there is a thread on here about them) regularly get that size without double yolks, but for a quail that hasn't been selectively breed for large eggs for generations, I think it would be caused by something messing with the egg production somehow. Perhaps hormones.