Here's a vote for the Koni-Omega/Rapid Omega cameras. Not small, but not too heavy. Absolutely superb lenses, though there are only four of them. With the Omega Rapid 200 and the Koni-Omega Rapid M (I think that's the correct name), you can swap backs mid-roll, if that's an important consideration.

The cameras take a little bit of getting used to, just because their controls are a little different (like pull-push film advance), but they are very rugged and extremely affordable.