That is all absolutely true, however... this particular case involves a hosting company that offers 100 x 100M databases, with no option to reallocate them as, say, 10 x 1G databases (or even 1 x 1G database, if you want to make the argument that it's easier to find a box with 100M available than one with 1G). Even if you're running them from a managed dedicated server. AFAICT, the only way they'll let you get a database over 100M is if you're on an unmanaged dedicated server, in which case they charge the same rates to provide less service.
So, yeah, I'd say it's completely arbitrary...