Panel 1: As we know, the sane and intelligent response to Mike's comment is to ask him point blank on what world does a fourteen year old boy get to tell a woman in her forties what to wear. Since Elly is neither thing, she looks down at her legs.
Panel 2: She then gets all defensive and stupid about how there's nothing wrong with her legs. Besides, even bigger women than her wear shorts all summer long.
Panel 3: He smiles and shrugs so as to ask her what her point is. This obviously leads her to assume that she has embraced being a fat, disgusting slob.
Panel 4: Since she cannot love what she sees in the mirror, she stands there horrified at how close she came to loving herself as she changes into slacks.
Summary: Mike might be a stupid pig but even he knows that the one thing that can shut Elly up is her fear of being a tub of lard.