Phoebe getting to the big point of the matter!
In addition to being able to elongate, which is linear, or one-dimensional expansion, Phoebe is able to expand in multiple dimensions, including in all three dimensions at once! She doesn't become more massive when she does this; instead her cells basically inflate like balloons. This does result in a general smoother shape, and some transparency when expanded nearly to her limits (as seen at the tip of her finger here), but this does mean that Phoebe is effectively able to increase the volume of any part of her body at will! Because of her lower density at that volume, this is mostly for show, since she lacks the strength or mass to do much with her seemingly larger appendage, but this does allow her to fill space with even a small portion of her mass, which can be useful for anchoring into things to get a better grip! She cannot expand her hair, however, since she has no direct control over it. Instead, when it appears to be different sizes, that is simply the result of it growing between her skin-molting cycles. Seems she has a molt coming up very soon, given how big her hair is in this one!
Gotta be honest, this started as just an excuse to draw Phoebe's ass. I did want to incorporate a non-standard stretch into it too, though, and that's where I remembered that I don't draw a lot of Phoebe expanding, so I went with this reverse-taper, both as practice drawing fluid expansions but also as practice drawing hands! I also continued on my Learning To Not Draw Stiff Poses Arc, and I think I'm improving even more with that! Finally I also decided that I should push myself to learn to shade the back parts of Phoebe's suit correctly, not just take the easy path and color them really really dark gray. I also wanted to work in her new shoulder emblem, but we can't get everything we want; such is life.