Can you clarify just what you mean by "compacting" them? How? Do you mean in the Comcast webmail interface?
When you reach the 10 Gb limit, anyone sending you an email will get a bounce saying "Mailbox full" or some such. At that point, you will need to start deleting old messages you don't need anymore. Especially those with large attachments. I would suggest you start house-cleaning your account now to avoid any problems down the road. You could also use a desktop client like Thunderbird or Outlook. That way you could store old emails in local folders in the client, and delete them from the server. There is no way to compress or "compact" the messages in the webmail interface.
I assume the 10G limit counts all your folders. You may want to look through Trash, Spam, and Sent to see what can be deleted. You can also sort by size, look at the biggest ones, and see if you really need to keep the full mail. Maybe just get a copy of the attachment and keep it on your computer or cloud drive and then delete the actual mail.