As Brian correctly states it can catch your boot etc. I have just bought the Wolfman front fender bag and my tools fit nicely in it. I would recomend using the front fender rather than the rear.
i have the wolfman tool bag. it bolts on to the rear fender. well the base does, then the bag velcros to the base and then straps from the base go around the bag and secure it. this bag is fantastic, as unlike most tail bags, this one is able to be taken off, and taken to the job. it looks tuff as also. highly recommend.