(Strip Number 4231, Original Publication Date, 25 September 1982)
Panel 1: We find ourselves watching Elly putting Christopher to bed; as she does so, he asks "Where Mama?" and she tells him "Gone to the hospital, dear."
Panel 2: He asks "Go get baby there?" and she confirms his suspicions by telling him that yes, soon he's going to be a big brother.
Panel 3: She smiles as he points to himself and asks "Bigger than this?"
Summary: The "joke" is that he's thinking that he'll become larger physically; this is an age-appropriate understanding of the situation that isn't being presented as an attempt to confuse or oppress Elly so it must be something that she's actually experienced. Perhaps this is what she said when they were explaining Alan's being born.