Perhaps "allocated" is the wrong word since it implies a fixed guarantee. But using the context of shared / common space, I will use a portion of shared / common space equal to or less than what the airline's guidelines say, which is typically one bag in the overhead bin, up to the published dimensions.

Traveling with a bag smaller than the published dimensions is hardly a selfish thing to do. If anything, I'm increasing the chance that someone later in the boarding process gets to put something else in the overhead bin that they wouldn't have if I'd traveled with a max-sized bag.

The logic that traveling with a smaller bag than allowed thus forfeits ALL access to the shared space is beyond comical.