Yeah, that short answer that UCI dictates the form is right. As someone who has ridden mostly outcast bikes for decades, it's interesting to see how uniform things are in "normal" bike land, aside from mountain, fat, and cargo.
Recumbents come (came) with different sized wheels in front and back not, I would argue, because of compromises in frame geometry, but in order to meet the different design goals recumbent designers and riders had. Freed from UCI- or mass market considerations, and not caring about fashion, recumbent builders did a lot of odd things to make what they wanted. Lots of it was home-build until the 1990s, and even then, very small production numbers from established makers.
Unless you wanted your feet really high (and many early designers clearly didn't and were looking for more of an upright, lawn-chair type position) then smaller wheel in front got that done.
Most common for a long time was a bigger rear wheel (559 or 622) in the back with a 406 up front. Tadpole trikes are still mostly built that way too. Short wheelbase recumbents were usually big/small or small/small until the late 90s early 2000s when 2x 559, or 571 (later 622) "hi racers" with stick-frame designs became popular. There are even some long wheelbase designs using dual 622s, and they are a trip to ride first time you try.
Personally, though I own some of all of these, I have settled on the small/small combo for almost all my riding and building. I like the snappy handling, light weight, compact design, and the ease of getting a really nice, low-end gear range with standard gearing on a 406 wheel. (I made some <23 pound CF touring bikes that will go in a standard suitcase and have an 18-100 GI range...) With 40mm tires, the ride quality is perfectly adequate on all but the worst surfaces, and a lot better than bigger wheels with skinnier tires (though not obviously big wheels with fatter tires).