Wonder what kind of MTBs that urbanvelo dude's been riding? My 15" (38cm) MTB has a 56cm effective top tube. I have had some short top-tubed MTBs from back in the day, but "serious" MTBs made after 1990 or so rarely have short top tubes.

Major toe overlap on this shorty from 1988 !

1988 Giant Iguana by Lester Of Puppets, on Flickr