It's actually Elly and Annie making a smug remark about their being pleasingly plump.
Panel 1: As they push strollers past a shop window, Elly moans to Annie about the bathing suits men expect them to wear that year.
Panel 2: Even if she were to lose ten pounds, she still couldn't wear something like that.
Panel 3: In what is clearly an altered panel, Annie console her by stating that the busty, wasp-wasted, thin-legged girls are all deformed....
Panel 4: ...while the two of them, with their fat, jiggling asses and wide, child-bearing hips are normal.
(Note: There should be a fifth panel that has Hank Hill hold the two of them at gunpoint demanding that they return his spare gluteal protheses.)
Summary: While I agree with what Annie is trying to say here, it seems to me that her misguided attempt to get Elly to feel good about who she is will be misinterpreted as license to beg off exercising and simply gorge herself on greaseburgers with extra bacon, tuna cardiac-arrest casseroles and home-made sundaes with three different kinds of fudge. It doesn't take a genius to see that Monday's letter will have an abridged version of this warped and misunderstood Aesop.