I'm pretty sure that we just haven't seen her other expressions. I'm saying this because pretty much every other character shown so far has shown a great improvement in facial expressions.
Or it could be...
all polygons meant for different facial expressions went to her booty. It's like when you take fat from your booty to fill your lips, just the other way around.
I made a comparison between Sheik and Zero Suit Samus:
Sheik is portrayed as athletic, not sexy. Sure, she has curves too, but her booty looks tight and compact, almost like a male butt. And look at her arms - they have visible muscle tone.
Samus is portrayed as sexy, not athletic. Her booty is big (just look how much bigger it is compared to Sheik's) and looks full, soft and plump. It's not fat, but it's definitely not firm either. Her arms are slender and thin, but lacks the muscle of Sheik's arms.
When it comes to the female form, athletic is usually not considered sexy. Soft, curvy and plump is considered sexy for a woman. So even though Samus logically would have an athletic build, she does not - because it would make her look less sexy.