why my iphone6 is storing too much on mail

I have an iphone6, and i started noticing that i was running out of avaiable storage. When i click manage storage it says that my mail is taking 3.4 GB.

I have Gmail, AOL (only 2 mails saved) and another one that keeps everything on the server and i delete everything on the phone always.

My iPad has the same settings and the storage Mail is just a couple of MB. How can i fix this? I only have 906 MB available.