Shipwreck, they're just goofy. The opposite tends to be true: Smaller wheels need larger corresponding gearing to be comparable to "normal" size bikes. Not unusual to see Bike Fridays, for ex., with insanely large chainrings to compensate for the smaller wheel size.
Now with rider weight, we're getting into a different issue. With standard non-folding bikes, rider weight tends to be harder on wheels much more than the frame. The wheels flex more under load and so very close attention needs to be paid to proper spoke tension, dishing, etc. Some believe that double-wall rims reduce flex and therefore prolong wheel life.
With folding bikes and increased rider weight, I'm going to say something a lot of folks here may not agree with. Stay away from cheap folding bikes. I lump the downtube and the citizen in that category.
Cheap folding bikes tend to have more issues with hinge clamps etc. so in my mind, not worth it to trust those parts to hold up to repeated stresses. Any of the better dahons, bike fridays, bromptons etc would be fine. Heck, I just bought a 90's Fuji folding mountain bike that would probably work in your situation, although it is certainly not as elegant as today's models. Pic attached.
Don
