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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.