Someone extensively covered in fat, though not to the point where the weight sags under the force of gravity, is termed well-upholstered.
Yes, just like a sofa:
American 1820s sofa. Photo by Wmpearl
Two thoughts occur. First, what a good thing it is that fat spreads itself so neatly over extensive areas of the body rather than appearing as a second nose, or a third ear; and, second, what's upholstery got to do with the sort of holster that holds a gun?
Well, nothing, actually.
Thing Probably Not To Be Today: well-upholstered. In the 1600s an upholster was a small furniture dealer. Why? Well, to uphold a belief is to give support to it, and furniture gives physical support to all of us (I mean, who isn't grateful for cushions?).
Holster is a different word entirely. It's Dutch, and is related to the Old Norse hulstr, sheath, and the Old English heolstor, darkness.