Getting rid of 9 GB's of saved messages?

Okay so, I have an iPhone 5S and it's a 16 GB and I'm already out of bytes. I have no clue how, but it's always popping something up telling me my storage is almost full and it gets really annoying. It says I have "Messages (Saved) 9.0 GB" and I'm not sure how I have that many, or I'm not sure how they even get saved! I have tried several times to delete all of my messages to see if it would help, and still no luck. It's so bad that I can't even download an app, or take a picture because it says my storage is so full because of the 9 GB's the messages are taking up! Any help is much appreciated, I have no clue how to do get rid of them and it would be so very helpful if someone could find the solution to this! It's irritating when I want to go take a picture and it says I can't because my storage is so full! I have to keep deleting apps because of this to fit more things on it such as pictures. I only have 146 pictures on my phone by the way! I really need help, and so yeah please help me!

Go to Settings > General > Usage > Messages, you can find out how much space it takes. Try to export or back up your phone to computer, then search and delete the multimedia files if any sent through messages.