Here's the issue: most folding bikes are compact in part because they have a low step-through frame and the rider is supported on a saddle attached to a very long seatpost. The seatpost acts like a long lever on the small seat tube. (And when there are frame problems on a folder, it is often because the seat tube cracks).

Most trail-a-bikes attach near the top of the seatpost, which, when combined with the weight of the rider, can potentially put a lot of levered force on the seat tube. Obviously, this is a bigger problem if you weigh 220 lbs and the bike has a weight limit of 240 lbs than it is if you weigh 140 lbs.