Let's tally the sales like Gladys and Patti LaBelle: I'm a big wig
Gladys Knight and Patti LaBelle are very successful R&B singers who have sold many records. They are also well-known for wearing wigs. The pun here is that he is a bigwig (a person of importance). And to add further meaning to the pun, in the music industry that often translates to record sales.
Patti Labelle wearing a big wig
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