Snoopy gets longer when he runs or lays down. Look at his appearance two days ago:
He has a much greater volume when he's in motion! It's easier to draw a larger animal when moving; it's hard to picture a little lump like Sitting Snoopy getting up and having a stroll. I expect that Schulz noticed this too, which may be why the dog gradually increases in size.
The strip itself is another on the theme of Snoopy steals something then eludes his human pursuers in some manner. Kind of light as far as gags go.
The "YIPE!" is a little interesting. Schulz has been using these outline letters for onomatopoeia for a while, and they, like the fancy question-marks, are a subtle trademark of the early strip. They're fairly striking.
