I used the medium bag most of the summer and filled it to the max. It held up fine. A riding buddy has the small version with minimalist flat repair tools and is happy with his. My LBS recommends using the light and getting a bag that can be used on any bike.
The medium bag was a little small for my taste but I was carrying too much stuff anyway so it forced me to go with what I consider the bare essentials but still twice as much as most people carry. BTW, my Alliante Sport seat ICS was defective and would not hold the bag securely. My LBS warrantied the seat and I swear that the bag is so secure you could pick the bike up by the bag. It certainly is not going to come loose with normal use.
You can attach an extra tube to the top of the bag to take up some of the gap that you do not like. It stays cleaner on top that it would attached to the bottom of the bag if you get caught in a storm.