You can find Federal guidelines for overbooked flights here:
http://airconsumer.ost.dot.gov/publi...tm#overbooking
This sentence applies to your situation:
* If the airline must substitute a smaller plane for the one it originally planned to use, the carrier isn't required to pay people who are bumped as a result. In addition, on flights using aircraft with 30 through 60 passenger seats, compensation is not required if you were bumped due to safety-related aircraft weight or balance constraints. *
Most airlines will compensate something for removing passengers due to weight restrictions. Legally, however, they are not required to do so. I think the majority of airline employees haven't got a clue, so they offer the same compensation as an overbooked flight. I would try to negotiate the best deal, but someone in the know will probably deny any more than what you got.