Well, those speeds may be determined by some or all of:
Vs
Vmca
Vmcg
Vmu
Vmbe
Assuming it's the same wing and tyres/brakes, I can't see that Vs or Vmbe would change (if anything, Vs might go down, assuming the same cg envelope, though I doubt they'd bother trying to take credit for it). One would expect the longer aircraft to have if anything a slightly better Vmcg/a (though these things can be odd) which only leaves Vmu as a candidate. If the longer aircraft are geometrically restricted over their shorter brethren, that would be logical.
edit: it's not mass, because those data are quoted at the same weight for both types. Unless Boeing are playing fancy games with minimum flight weight on the larger aircraft in order to reduce Vmc's - again, something I doubt