Same size film and backing paper as 120, however the 620 spools are smaller at the core and to the edge.
A Kodak developed size that:
1) permitted a slightly smaller size camera; and
2) "encouraged" people to buy 620 Kodak film instead of competitors' 120 film.
The Kodak Brownie Hawkeye Flash and the Medalist are two of the best known 620 cameras, but there were lots at one time.
There hasn't been a new 620 camera for decades, and the various types of colour and black and white 620 films haven't been made for years, but as many of the 620 cameras were both good and very durable, people do various things to permit them to use 120 film in their 620 cameras.
I have two 620 cameras - a Kodak Tourist and a not yet tried out Kodak Brownie Hawkeye Flash.