What a strange assertion. K and DBT are just as popular as KS and C, both among pianists (who often record them in pairs) and the audience who fill the seats of any such recital. They are not much longer (actually, depending on performance, they can be shorter than C) and their melodic appeal is similarly immediate.
I don't have exact statistics but I'd bet a reasonable amount that Kinderszenen is BY FAR the most popular piece of the ones mentioned. Not only Träumerei but altogether it's almost the only Schumann piece that is popular in the sense of Wedding Marches and Hungarian dances. And I am also quite sure that my order of popularity is roughly correct for the other ones. As for length, I was unclear, I meant not the overall length of the cycles but more the lengths of the pieces contained.
In any case, bracket DBT and some others as borderline cases. I think there cannot be any doubt that there is a large popularity gap among different pieces of Schumann's piano oeuvre. That Demus box with solo piano has 13 CDs. I'd say, the popular pieces are about 3-4 discs worth, another 3rd is in the middle and the last 3rd is quite obscure (or piano lesson stuff like Album für die Jugend). (Of course, pieces are mostly "mixed" in that Demus box, so that's a rough estimate.)
And a lot of this music is not at all like Mozart's, immediately appealing and therefore comparably popular, even in case of lesser known pieces. Humoreske is an odd and "awkward" piece by almost any measure, I'd say. It's never going to be as popular as Chopin's 2nd sonata or Schumann's Symphonic Etudes.