The reason is that big wheels do not have a major advantage for a folder (and even a non folder).
What makes the comfort is the wider tires and suspension.
The lack of comfort of the Brompton (probably its biggest drawback) comes from its high pressure, relatively narrow (35mm max) tires and lack of suspension (the rear suspension is very poor and there is no front suspension, actually the front suspension is the most important for comfort).
If you are looking for a compact and comfortable folder look at the Riese & Müller Birdy: it is a full suspension bike (front and rear) and accepts 50mm wide balloon tires (Schwalbe Big Apple).
You can try it at "Les vélos Parisiens" they sell both the Brompton and the Birdy.
I have both a Brompton and a Birdy, the difference on bad road and cobbles is as big as between night and day !
If you are looking for a lightweight less expensive Birdy, Pacific Cycles has other variations of the Birdy with other components than Riese & Müller and it now should be possible to buy it online from the Pacific Cycles online store with shipment to France.
For the wheel size, there is no real difference between ETRTO 349 (Brompton), ETRTO 355 (Birdy) and ETRTO 406 = 20" wheels. BTW, Pacific Cycles has a sport 20" wheels Birdy (the Birdy frame accepts 20" wheels with tires up to about 30mm).
Montague are much bigger folded and relatively heavy.