The sprites weren't taller at all in the Genesis game. They only looked taller because of each console's native resolution.
For Mortal Kombat 1, the sprites for both games are draws at roughly the same pixel resolution:
But because the Genesis native resolution is at 320x240 and the SNES's native resolution is 256x224. The SNES sprites look much wider and shorter when displayed at 4:3 on an oldschool TV.
SNES:
Genesis/ Mega Drive:
For Mortal Kombat II Sculptured Software tried to correct the wideness of the sprites at the 4:3 aspect ratio by making the sprites skinner.
Genesis sprite on the left, SNES sprite on the right:
SNES:
Genesis/ Mega Drive:
The SNES sprites in MKII actually lose pixel detail horizontally. But it was done to make the sprites look proper when viewed on a TV.