Mobility i s a spectrum. People like Echo are much closer to immobility on that spectrum than, say, Mochii. She is closer to the immobility side than, say, me. On the other hand, professional athletes, dancers, and soldiers tend to be further on the mobility side than me. It is not all related to weight. Boberry is heavier than Mochii, but seems to be more mobile. Being "truly immobile," as in pinned to the bed due to obesity is exceedingly rare. There was Carol Yager, that Egyptian chick, and, well, that's basically it off of the top of my head. For all intents and purposes, Echo is immobile. Gina was immobile. Vanilla Hippo was "mobile," but that mobility was extremely limited. I think it's much more useful to describe levels of mobility when talking about real life humans than to just talk about fully immobile vs not.