I can’t say how logical it is, but lore-wise the suit doesn’t actually frame her body proportionately, it’s larger and her feet only go to the shins and her arms only go halfway down the suits arms. The way it moves without 1:1 mechanical input from her body is assumed to probably be related to the “biological” aspect of the suit inside that is made of living tissue and is essentially an extension of her nervous system. Also alien-bird-DNA
This also ties directly into the plot of Metroid Fusion, were her encounter with the X parasite caused the biological components of her suit to fuse with her body. In order to stabilize her, the alien-bird-dna already fused with her human dna was further fused with metroid dna. Fusion! Fusion! Fusion! Do you get the title yet? Fusion!