I am unsure if anyone else has observed this; the other day I went to Settings>General>Usage. I noticed that the stock "Messages" app was "using" 786MB on my phone. My immediate thought was: ok, I send a lot of group MMS' to my family and all, so I could maybe see how all the messages I have on my iPhone are taking up that much space. So, I went ahead and deleted all of my the messages in my inbox. I checked the usage on my phone again, and it still read 786MB. I restarted my phone ... still at 786MB. I have a 16GB (iPhone 5S FWIW), so 786MB is of higher value to me. Any thoughts on how to clear this up? Thanks!