The show writers thought they came up with the word on their own, they didn't know about the previous usage of the word in 1884 (the episode was written in 1996

The show writers thought they came up with the word on their own, they didn't know about the previous usage of the word in 1884 (the episode was written in 1996, the internet wasn't quite as full of facts back then), "embiggen" was still supposed to be a joke.