Not surprising when you consider that on the V-280 the fuselage only needs to hold the crew and passengers.
Compared to the SB>1 fuselage which must house engines, gearboxes, fuel, in addition to crew and passengers.
I was referring more to the width of the fuselage. While the V280 fuselage is narrower than the UH-60, the SB1 appears to be much wider. Length of the cabin obviously can't be determined from the overhead shot.
So it might be that V280 has a long but narrow cabin while SB1 has a wider but shorter one.
It seems I haven't seen cabin dimensions of the two so far. Any numbers out there in the wild?