Noted as “The first atypical size Garfield comic strip” over here: https://garfield.fandom.com/wiki/Garfield,_December_1980_comic_strips

No explanation though. Tried searching online, and Google’s AI was once again unhelpful:

There’s no specific “atypical size” Garfield strip for December 21, 1980; that date marked the famous John Lennon murder, so any special Garfield strip would likely be a tribute, but most online mentions focus on Garfield’s usual antics, with no physical size anomaly reported for that day, just the news context around it