My guess what’s happening is:

-it’s a later post-cancellation announcement wave so less were produced (it’s completely delisted on Big Bad which means it’s beyond sold out unlike so many other waves)

-a 7 inch scale fat Homer will only look plumper and fatter when surrounded by 5 inch figures. I believe other characters who could blend in (Devil Flanders, Radioactive Man) had a lot of issues so he might be the only one “worth saving”

-the Itchy and Scratchy robots also work but wave one was MASSIVELY over produced so they’re just giving those way…

Someone probably posted somewhere on social media a picture of King Size Homer next to the 5 inch Homer and people thought it looked good and bought the remaining retail stock, collectors with a little extra money to burn turned to eBay and resellers who got burned buying ULTIMATES! noticed an uptick in just Homer and seeing all retail stock was gone and realizing they were the last remaining option to get this figure, FINALLY, massively raised the price, because you miss 100% of the shots you don’t take and if this figure was selling fast for some reason other than “oh that’s cute, guess I get…” you gotta jump on. They’re LISTED for $400+ not necessarily selling for $400+,price will probably normalize and drop below retail once King Size Homer gets a 5 inch figure.