I think it means that, although she is heavily-built, she is not very tall. A banquette is a built-in bench seat against a wall or in a corner, with a free-standing table. Here's a pic of the seat part. "Winkled," I figure, means crowded in behind the table.
You forgot the pic of the seat part, but "I get the picture." I don't know what this woman's overall shape is, but if she is short and carries most of her weight below the waist, she could look not
quite so obese if she's seated behind a table.
She could try gastric bypass. I got a RNY in 2008 and on the evidence, I am convinced it saved my life.