First the good news. According to Haas, texts sent through iMessage take up very, very little space relative to the storage available on an iDevice. But texts aren’t the only info being sent back and forth via the service. “However, the attachments– usually pictures, videos – sent through iMessage take up the vast majority of storage used by iMessage,” Haas says.
“The Phone has an option that will show how much storage each app is taking relative to other apps and available storage capacity of the device,” Haas says. “[…] The device even recommends the best way to reduce the storage used by iMessage – delete old conversations and attachments – and how much storage will be freed.”
Thus, one of the most important settings to adjust is iMessage storage so that some of the messages taking up the most space (including those with attachments) are deleted and not stored on your device.
Of course, you can always delete individual conversations, focusing on the ones with attachments, but doing this manually will take up a lot of time. Haas recommends the more efficient process or scrolling to Messages, tapping it, and then under Message History, tapping on Keep Messages. From here, you can select one of three options: 30 days, 1 year, and Forever. The 30-day and one-year options ensure those messages are eventually cleared off your phone without you having to do additional work.
Keeping an organized handle on your iMessages and not allowing them to get out of hand (especially if the people in your universe love to send you attachments) is one simple way to keep your device free of clutter and working more efficiently.