We are told that "Mrs. Croft, though neither tall nor fat, had a squareness, uprightness, and vigour of form, which gave importance to her person. She had bright dark eyes, good teeth, and altogether an agreeable face..."

In other words, she's a fine figure of a woman, healthy and hearty (albeit with somewhat weatherbeaten skin, not having had the benefit of sunscreen). Presumably Sir Walter finds all of this a bit coarse, preferring a more effete look, the Lady-Bertram-lounging-languidly-on-the-sofa type...