Today my Gmail account read as "You are currently using 6538 MB (88%) of your 7429 MB.". Now that was a bit worrying for me and decided to bring it down atleast by 5% and here's what I did for it.
Head over to the search bar on top of your Gmail account and type as "has:attachment (*.mov || *.mp3 || *.wm || *.avi || *.wmv )"
and the results you get could be the large file attachments that are sitting in your GMail account and making it bulky. Evaluate if you need it else you can select them and delete.
To find large zipped attachments use this search option.
has:attachment (*.zip || *.rar || *.7z )
To find large pdf / doc / xls / ppt / docx / pptx / xlsx attachments use this search option.
has:attachment (*.pdf || *.doc || *.docx || *.xls || *.xlsx || *.ppt || *.pptx )
Usually ppt / pptx are large culprits too.
P.S: After you delete the mail and attachment, it still remains in your GMail Trash folder. So go over to that folder and hit "Empty Trash". Now you can finally see some very good increase in the storage area of your Gmail account.
As for me, it went upto "You are currently using 6138 MB (82%) of your 7429 MB." which was a pretty good increase for me.