Decrease size of Comcast email folder

I use Xfinity email and access through Outlook client on my laptop.  My email with Xfinity is 9+ GB and is reaching the limit.  Synchronization to my laptop is really slow and my incoming email takes a long time. 

If I use the Archive feature in Outlook, will this reduce my Xfinity mailbox size?  What is the best way to reduce the size of my mailbox? 

Does Xfinity have an Archive feature to decrease the size of my mailbox without going into each folder and manually removing large files?