A full-size wallet such as a Zippy will fit comfortably inside plus other larger items like a Cosmetic Pouch, agenda, Cles, a larger phone, etc. You may have to get a little creative on packing from time to time though.
It definitely will fit a full size wallet but I always use my zippy compact wallet or zippy coin purse with my Speedy 25's. It's just a lot easier for me to get my wallet in and out of the bag when it is full of items when I downsize my wallet.
I have never had an issue or annoyance with my full size wallet. I place it in at a slight angle, but the opening isn't narrow so it's not an issue. I also carry 2 small cosmetic pouches, 2 phones, 2 sets of keys on a daily basis. I also add two 16 oz water bottles for work and there is still room!