I had difficulty deciding between this Topeak seat bag and the Avenir Bigmouth Velcro Seat Bag (Large- 73/106 Cubic Inches). I also had difficulty deciding between the medium and large size Topeak seat bags. I wanted the largest seat bag I could get but I didn't want the chance that it would rub on my legs while pedaling. For certain, Topeak's website and catalog have a dizzying array of seatbags. I would have preferred the Aero wedge that has the "fixer" (the plastic connection to the seat), but I read far too many reviews stating that the plastic breaks far too easily.In the end, I'm glad I chose the large Topeak seat bag for my Trek DS 8.5 hybrid with stock seat. It fits perfectly under the seat and doesn't interfere while pedaling (see update below). The straps hold it very tightly to the underside of the seat and to the seatpost. The bag also has flexibility that allows it to be shifted in a downward direction on the seatpost rather than hanging the bulk of the bag from the bottom of the seat. For example, since my seatpost is rather high, I can shift the bulk of the bag onto the seatpost. I think it's better than the Avenir Bigmouth bag for one reason: The Topeak's cornucopia shape allows it to hug the seatpost a little better whereas the Avenir Bigmouth, to me, just looks like a big chunk hanging underneath the seat.The bag has a zippered expandable section. With the bag in its fully open position, I can fit the following items (see my additional uploaded pictures):IceToolz Steel Tire Levers Set, 3 Piece2 valve adapters (Kool Stop Brass Card/12 PV Valve Adaptor (Pack of 12)) inside the patch kitInnovations Air Chuck Elite Inflator including sleeve and CO2 cartridge3 additional 16-gram CO2 cartridges3" Pocket knifeReplacement 29er tire tube2 AA batteries in plastic storage case for my GPS4 AAA batteries in a plastic storage case for my Portland Design Works Radbot 1000 1W LED Tail Light2 CR123 batteries in a plastic storage case for the Fenix PD30 flashlight/headlightFenix PD30R2 6 Level 235 Lumen LED FlashlightBeretta 84FS small frame pistol in holster (biggest item by far in the bag)Park Tool VP-1 Vulcanizing Patch KitDriver's license & other identification and a little cash in a tiny envelopeThere's a bit more room to spare for a couple more items the size of CO2 cartridges, but there's no room left for a sandwich, wallet, or snack bar.One minor gripe. With the bag tight against the seat bottom, it's a little difficult to get the zipper open. Other than that, I'm so pleased with the bag that I also bought one for my wife's bike.Update: After getting more comfortable with the bike, I decided to adjust the stock seat forward for better posture. My legs now do touch the bag a bit while pedaling since I'm not able to move the bag toward the rear as far as I used to. I guess your mileage may vary depending on how your seat is positioned, and different seat bags that hang further back rather than snaking down the seatpost are probably less likely to interfere while pedaling. See extra uploaded pictures for a "top down" view of the bag under the seat. To solve this problem, I put two heavy duty zip strips at the appropriate height on my seat stem. This prevents the bottom velcro from sliding down the stem further than I wish. It works perfectly and prevents the bag from getting in the way of my legs while pedaling.Also, be careful when carrying a lot of weight and attempting to use this on mountainbiking trails. After carrying the items listed above on intermediate singletrack for about 15 miles, the rear seam of the bag ripped. (See additional uploaded picture). I foresee no problem when using this bag on smooth trails, but--depending on the weight you're carrying--it might not be sturdy enough for offroad trails. I repaired my bag with heavy duty black quilting thread and just kept on trucking.