Its not the "towing" that you need the size for its the STOPPING. That's where a lot of people loose control. The other instance in a bumper pull is that it starts swaying behind the vehicle and it creates enough momentum to jerk the back end of the vehicle around....look at how many people tow those giant mobile homes with small trucks....and are wrecked on side of the road because the trailer got to swaying and the truck didnt have enough weight. Hauling live animals adds sway and movement to any trailer, and that is all magnified when it goes through the hitch and into the rear of the vehicle.
I tow something somewhere everyday, I live and work on a huge farm. Something is always hooked to me. I have a two horse bumper pull and I always pull it with my 3500 duallie or my 2500 diesel excursion or I have a 3500 suburban I put a Cummins in and beefed up everything
in it. It does great towing and weighs 8450lb on its own. It has enough "butt" to hold anything I ask it to tow with a bumper hitch. Id rather be 'overkill' than underkill when my babies are back there.