Paul ... trailer bounce is more a function of wheel base, length of trailer a-frame, and highway expansion cracks than it is towing a tiny trailer.
Even my good friend with a 30 foot Jayco experienced this phenomenom on certain roads.
Now, I'm no engineer, so don't expect any technical explanation, but for what reason, certain tow vehicle/trailer combos, running on certain highways with certain highway expansion cracks, can develop a simple harmonic motion, and bounce up and down vigerously.
The solution is easy ...
1. Don't run down the roads that cause you problems again! (Hey, don't laugh. My friend winters in Texas ... and cutting off a corner on the trip on Highway 69 in Oklahoma, always bounces him around. He now runs into Tulsa and turns left).
2. Change the dynamics of your set-up. Try using a longer ballmount in your receiver .... they make long and short ones. If that fails, and your trailer tongue weight
allows it, try adding a 12-inch extension between your drawbar and ball mount. By altering the length of the distance between all of the axles, you can often reduce or eliminate the problem all together.