How to Free Up Space Used By Your iPhone or iPad’s Messages App "If you send and receive a lot of messages, the Messages app can take up a lot of space on your iPhone or iPad. Not only does it store your message history, it keeps photo attachments you’ve received. All this data then takes up iCloud space as part of your backups, too."
After doing yesterdays Apple updates I checked my storage usage and found iMessage was one of the biggest hogs of storage on my iPhone and iPad. I was keeping messages forever and some friends send a lot of pics including some videos. The tips here helped me free up some space.