This is one of the "duh" things, but it needs to be experienced to appreciate:
I have two bikes: a folder with 20" wheels and Surly LHT. During the winter the
folder experienced the most use - commuting to work. When going down a hill
on the folder, at 40 km/h one is thinking about writing his last will, at 50 km/h
one reviews his life in a flash, and at even higher speeds...what higher speeds?
Yesterday I took the LHT for a longer country ride (the first of this year) and when going down the hill at 55 km/h the ride was so solid and stable it didn't feel fast.
I don't want to put down the ride of a folder. I enjoy riding my folder even in a hilly terrain on longer rides. Just the difference in the stability and ride solidity is so profound...

It's probably a matter of geometry, and specifically wheelbase. I bet the folder has a *really* short wheelbase. In fairness, it's not designed to go 50 kph; it's designed to stash in the trunk and ride around the campground.

I agree that the shorter wheelbase is a contributor, but combined with the 20" wheels vs 26" wheels the difference in overall stability would be dramatic. Smaller wheels spin faster and inherently steer quicker due to their smaller radius. No bike with 20" wheel is ever going to hold a line or maintain the stability of one with 26" or 700c wheel.

Steering geometry and overall solidity of the construction probably play a bigger role than wheelbase. My 20" folder feels very stable at either 50 kph or 50 mph (but I don't often reach the latter). Its steering is a little quicker than my 700c touring bike, but not as quick as my 700c crit-geometry road bike.

My folder is designed for loaded touring not riding around the campground, although it can do that too I'm not sure how fast it can go downhill without feeling unstable because I don't really fly down the hill on a loaded bike.