I not only focus on rolling resistance, there are other advantages for a folding bike and a not folding bike.

For a folding bike, 20" wheels allow to have wider tires that not only roll better but roll smoother on bad road surfaces and of road without having a too big overall wheel diameter that increases the folded size (I am pretty sure that Helix limited the tire width to 40mm to also have a not too big overall wheel diameter and keep the folded size small, the
Helix fork for instance has almost no clearance above the wheels).

Another effect of smaller wheels is reduced wheel inertia that gives a more reactive bike that accelerate faster which is interesting for city riding with a lot of speed variations due to crowded roads and lots of traffic lights.
Lower inertia is also good when there are many uphills and downhills, race road bike uses low rim height and low weight wheels for mountain races to reduce inertia and high height rims, heavier wheels for flat high speed races where cyclist are riding in grouped.