The wheel size doesn't define how a bike rides.

24" wheels (BTW., which 24" size, ETRTO507, ETRTO520, ETRTO540...?) do not necessarily ride better nor are necessarily more comfortable than 20" on bad roads, bumps... the most important is the tires and especially the tires width, wide tires (50mm or wider) with 20" wheels are better than 24" wheels with narrow tires.

For the riding behavior and bike efficiency, its the bike geometry and frame design that define them, not the wheel size.

For your test of a Brompton, unless it was the recent G-line with 54x406 wheels, its not a 20" wheels size bike, its a 16"-ETRTO349 wheels bike and the 16" Brompton is known to be very uncomfortable and very inefficient on bad roads.