Empty mail app takes 500 mb space. :(

My mail app had been growing in size (was at about 690 mb), I was advised to delete al my mail accounts (Gmail, exchange, outlook) and add them again, but there hasn't been a drastic reduction in the size of the mail app. It's still at 498 mb. I haven't added the accounts back yet. Doing a hard reset seems to make no difference. What do I do?