Have you at anytime analysed as to how much storage space is being provided in the Gmail being used? Have you checked how much storage has been used by your Gmail, Google Drive and Google+ accounts? This is not at all a difficult process, all you’ve do to view your utilization of storage is scan the lower left side of the Gmail page, it will be shown in % of total storage used.
Totally 15 GB storage space is provided freely for everyone. For example, your utilization may appear as ‘0.17 GB (1%) of 15 GB used’, click on the Manage option found below to know in detail as to how much storage space is being used by different services. Here you can view the usage of storage space through circle graph. 
If more storage space is required you can purchase in premium account. Beside the default plan that gives you 15 GB storage space for free, there are premium storage plans starting from 100 GB at $4.99/month to 16 TB at $799.99/month. The total storage space is shared across Google Drive, Gmail and Google+ Photos.