Sleek
Yes, a quick but ultimately tardy check with the dictionary reveals that you're quite right - one can be "fat" and "sleek" at the same time. (I guess that's how some of the chubby co-eds around here manage to retain a certain attractiveness to some of their classmates, although certainly not to me.)
"Sleek" means smooth and lustrous; healthy or well-fed; polished or smooth; slick
Live and learn (to always double-check with a dictionary first.
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