I got my Gmail account during the early beta invite only phasis, and the mailbox size increased ever since (remember the fancy mb counter, eh? 😉 ). Drive and other cloud backup storage hadn’t been invented yet, and sometimes sending mails to yourself were the best backup ever. Other than that, it’s just the increasing size of documents, scans and images causing my mailbox to grow continously.
Once in a while, you may want to move/clean those files. Using the gmail web application, type the following to search for size greater equal 5 MB and older than 1 year.