The film script strongly implies that because Stanley has so little personality of his own, that when he's imprinted by Loki's mask the extent of his transformation is near-complete. This evidently includes both his mental and physical appearance.
By comparison, both Dorian and Milo put their own stamp on the mask-transformation through their strong-willed personalities, only changing partially.
THE MASK CREATURE
He's dressed in a snazzy zoot suit - a distortion of the paisley
material of Stanley's pajamas.
The head is no longer Stanley's. It's large, bald and bright green. The huge bug-eyes glow with mischief. The nose is small,
bony and beaked. The mouth and teeth are enormous and gleaming white
as he breaks into a leering grin.
The overall effect is devilishly loony, but not altogether unhuman. In fact, there's something downright charming about him.
The Mask checks himself out in the mirror and likes what he sees.
as compared with
With a CRACK of thunder a whirlwind of light and power swirls around
Dorian's figure. Unlike Stanley's transformation, Dorian's is much
more diabolical. He grows and changes within a nimbus of ROARING
light. Finally the light dies away and Dorian/Mask rises from a
circle of swirling smoke.
While Stanley was a zoot suited bee-bopper in hyper-drive, Dorian/Mask is more like a hulking evil GENIE, fresh out of the lamp
and pissed at the world. His diamond earring and touches of his
neuvo-gangster look is still apparent, but his huge grin stretches out
like a TYRANNOSAURUS Rex's under eyes that glow green with wicked
power. His voice is a deep inhuman RUMBLE.