Why does my Messages app on my iPhone take up so much space?

I have an iPhone 5 on iOS 7. I've deleted all except 3 of my messages, yet it is taking up 1.3 GB of my phone's space. Why is it taking up so much space and how do I fix it?

Delete the other 3 of your messages.

The messages probably contain photos or videos, which can make them very large. Ordinary text messages hardly take up any space at all.

Did you fix this? I've deleted al of my messages, and the app still takes up 1.3 GB of space.

It's not just that the app takes up so much space, but the "usage" utility says it's the messages themselves taking up space (in my case, 950MB for 5 small basic txt messages with zero pictures). Makes no sense for the messages app to take up that much space for just 5 messages. Something's not right.