You may well find a single trailer built heavy duty enough to hold a small tractor, However it is not always the payload you have to consider, They're many other factors, such as it having no brakes, difficult to know where and how to distribute the weight? yes you might start out thinking you have the weight of the tractor perfectly balance over the axle however that weight can suddenly be transfered to the rear of the tow vehicle when having to unexpectedly hit the brakes, The momentum and extra weight will slam the rear of the vehicle down resulting in lifting the front of the vehicle and will not have adequate braking, remember the largest part of your braking comes from the front brakes of you vehicle, The next scenario could be while towing up an incline the weight then shifts to the rear resulting in tongue lift and lifting of the rear of the tow vehicle then loosing traction, and now fro the 3rd scenario, (Jack-knife ) is always a possibility in towing both dual and single axle trailers, But at least with having a tandem axles the weight is over the TWO axles and will have a safer chance of getting your rig stopped,
If reasons for think and wanting to use a single axle trailer are to prevent over loading the tow vehicle, Your wrong, You should use a lighter built tandem if need be instead of a heavier built single axle trailer, The rule is, It's better to have More and not need it, than to need more and not have it,
You can find smaller 5x10 ft tandem trailer, actually I had one and sold it due to obvious reasons of needing a larger trailer,
