Started out with the small, fit a tube&tools. No room for my Airbone pump!
So switched to the medium. Now I can fit the pump and even cram in my flip phone. Used an old black shoe lace put through the loops on the bag and tied up under the seat for added support after reading other reviews. After a few rides everything seems to be fine - a small tight solid package - don't even know it's there. Go for it!