Oh... that’s the vacuum cleaner bag she stole from the Kirby. A little like Scarlett O’Hara’s curtain-dress, but much uglier.
The Hildebeast has all her clothes custom made.
The only place she could find anything ready made to fit her would be in the tarp section at Harbor Freight.
Notice that she had to abandon her Pant Suits and move up to voluminous Mao tunics and humongous caftans as she got wider, lumpier and dumpier.