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This is a photo that ran in newspapers - it used to be that chubbier infants were considered healthy and would often be entered in 'Bonny Baby' contests, such as in this British Pathé newsreel from 1931, which showed a baby that looks about one year old with the caption "Fair, fat - and forty - (pounds!)"
In early posts of this photo, no mention is made of Gacy, like this Got Funny Pictures entry for July 8, 2014, entitled A Little Tony Soprano in the Making or this entry from Imgur date September 15, 2016, If USA was i (sic) child it would be this little fat bastard .
It may be that someone thought that this looked like the adult Gacy, who was obese and had gray hair, but real photos of Gacy as a child show him as having much darker hair and being thin:
And unlike the fat baby, he looks very unhappy - he was abused physically by his father, according to his sister.
While on the subject of miscaptioned photos, this photo that's been floating around a few years is not serial killer Charles Manson:
Image via Imgur
The baby's distinctive ear shape is quite different from that seen in Manson's childhood photos.
This was first noticed by Matt Novak writing in Gizmodo, who check with Manson biographer Jeff Guinn, who said the photo "in no way matches the verified photos I have of Manson as a baby."