How to clean up the mail on iPhone?

Dear all! Yesterday I realized that the Mail in "Statistics" on my iPhone 4S with iOS 7 occupied about 1.7 Gb in memory. At the same time two or my mailboxes (inbox, sent, outboxes etc) were completely free! I suppose that these orfains are somewere in the cache.

So how can I clean my iPhone, how can I delete these files? I found only one way - to delete mailboxes and to establish them ones more. After that Mail became about 76 Kb. But this way is so stupid! Maybe Apple provides us another solution?

Best wishes.