I think a "hobby" can have a wide range of involvement...a loop under a Christmas tree/small collection vs a year-'round layout/major collection... or someone who restores/owns old cars vs someone who has car books/goes to shows.
I belong to a Yahoo group related to the collecting of 3-D photo slides that were taken in the 1950s. Although there are approx. 80 in the group, only about 12 of us actually collect them.