As such attack vectors became apparent, successful mailbox providers realized the folly of allowing "a few kb / a few mb mailboxes," and limited mailbox size to a few hundred bites.
Initially it might have started uncapped - I actually remember having one that was uncapped, although I didn't use that one - until they had to impose the few kbytes limits due to practical considerations.
Email account caps grew and grew.
...as technology allowed it.
Initially, it was up to the sysadmin of whatever uni you were at.
Perhaps some providers did limit account size as a temporary anti-spam measure, as satoshi did with the blocksize. Technology certainly allows removing that kludge now, so let's not become the next AOL.