It's just snug enough to fit my phone(Samsung Galaxy SII), keys and wallet(moderate sized). It's small enough to not overwhelm the frame of the bike.
This is a small bag indeed, so watch out if you're the type to have one of those big wallets or extremely large set of keys with a handful of key-chains, IT WILL NOT FIT.
I don't know how this won't fit in other people's bicycles. I ride a Schwinn Prelude BBWhite. It fits perfect.
I did not give this a 5 star because I don't see a point in that hard piece below the bag arching across from upper left top to lower right bottom.
The corners actually scrap my leg if I get too close. But it's manageable and it only bothered me for the first 10 minutes of riding, I haven't scrapped my leg on it since then.
I think is a great pouch for anybody that needs a place to keep their phone/key/wallet, maybe a tiny first aid kit or tire patches or spare tube.. nothing else fits in there really.