A non-Newtonian goo to keep those proportions. Not that I am complaining.

Certainly explains how she's able to compress herself into a ball.

Be great if she ditched the power armor and starting running around as a slime girl. But this is Nintendo we're talking about, so I guess this series should remain family-friendly (melting poor alien animals alive in acid until they're nothing but a bleached skeleton notwithstanding).

Like half-goo-girl, half-mecha. Since canonically her armor fused into her body in Metroid: Fusion, which I believe is the last chronologically since Metroids still exist in all the other games.