What size is your bike? I don't have any problems installing the large handlebar bag, even with a fork-crown-mounted headlight (see here, but then, I use a 23" tandem. If your bike is small, you may find it interferes with headlights and other stuff.

What do you want to carry? I find the large bag perfect as a camera bag, as I can carry the full-size reflex camera, plus an extra lens, plus a couple of other items like maps, pen, paper... Or alternately I could carry a spare sweater, rudimentary rain coat and some food. On the other hand, if you only want it to keep your maps and finger food, the small bag will be more than enough.

The small bag is also much lighter to carry around. That's a good argument in its favour if you think of day hikes on tour.