In the early days with Emily I only used one booster, not both, with my bbh. She was in a small cover for a couple of weeks, but grew too quickly. It's not really until they get older, wee more, and possibly sleep through the night that you need to boost heavily.
They do look bulky, but they will fit. Depending on the size of your bub, you might find in the first couple of weeks he might be too small for them, but as you're using sposies in the first couple of weeks, you should be alright.