The IPM Bin folder is where all the incoming and outgoing messages are
saved. (Without a server the messages must be stored locally.) If you
start deleting items in the Out Tray and/or In Tray you'll see that the
files with the number names will be deleted.
Although the computer may be sitting idle, it can be receiving mail
which would create a new file in the IPM Bin folder. This accounts for
an increase in disk space used.