

Every barrel you see on TV or in a movie is 55 gallon or 42 gallon.
Other sizes exist but those two are so prevalent that you really only see those. The size comes from the oil industry where it’s a standard unit. It’s common to sell oil in different units, but the barrel size is so common that everyone just uses the same container and maybe just rounds the units.
Can’t use a 42 gallon barrel , has to be the metric 160L, or the 200L 55 gallon drum.
https://www.unileverfoodsolutions.us/product/hellmann-s-extra-heavy-mayonnaise-4-gal-1-pack-1-EN-639543.html
https://www.webstaurantstore.com/hellmanns-real-mayonnaise-24-gallon-drum/125HLMN8828.html
https://www.unileverfoodsolutions.us/product/hellmann-s-extra-heavy-mayonnaise-tote-2300-pound-pack-of-1-1-EN-1210411.html
The largest sold to consumers through normal channels is a gallon. Typically used by people who are feeding a lot of people, like making a dish for a large family gathering, or by people who only go shopping once a month or less. Some people live an hour from the store, so they just buy an excessive amount of food and shop infrequently.