early in the movie, the mother and father have a disagreement about which attitude towards giving kids sweets results in them being overweight, then when time moves forward a punch line is made out of the kids being much larger. there are several more jokes made about the kids' weight during the portion of the film where they are that age, including by the narrator. when the kids are older, after more time passes, the fat jokes end as the characters are shown to no longer be of size.