Panel 1: As one could expect, Elly asks the sales agent if the skirt she's trying on comes in a smaller size. Connie tells her that it looks perfect right now so as to imply that there's no need to do so.
Panel 2: Elly jabbers about that phantom excess ten pounds she mistakenly believes that she needs to lose when she says that there's no sense buying something too big. Connie sets up the joke by advising her to take the skirt in.
Panel 3: Elly's response is to say "Come on, Connie....you know me. How likely am I to take in a skirt" (thereby reminding us of the premise of Elly being a busy mother who gets no help and no time to herself).
Panel 4: Connie laughs when Elly reminds herself that taking in a skirt is as likely as losing ten pounds (thereby reminding us that Elly's life is one of endless woe).
Summary: Today, we get a passive-aggressive slam at the expense of the unsupportive family that conspire to keep Elly fat. Tomorrow, we get to see why it's unlikely that she'll lose her arbitrary amount of weight tomorrow when we're reminded that Elly thinks that pile-high helpings of 'healthy' food are slimming.