How to do it
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See how big Messages is
Open Settings, tap General, tap iPhone Storage, and find Messages in the list or the bar at the top.
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Open a conversation’s attachments
In Messages, open a conversation and tap the contact or group icon at the top. Tap a category such as Photos. To see one kind only, tap the filter button and choose Screenshots, Videos, or Photos.
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Select and delete
Tap Edit, tap Select, tap each attachment you want to delete, then confirm the number you are deleting.
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Empty Recently Deleted
Deleted messages and attachments stay for 30 days. In the conversation list, tap the filter button, tap Recently Deleted, select the conversations, and delete them. This can’t be undone.
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Let old messages clear themselves
Open Settings, tap Apps, tap Messages, scroll down and tap Keep Messages, then choose 30 Days or 1 Year. Everything older, including attachments, is removed on an ongoing basis.
Why is Messages so big?
Apple groups Messages and their attachments as one storage category, so photos, videos, and screenshots people send you count there, not in Photos (Apple: How to check the storage on your iPhone and iPad). Photos and videos in your Photos app are a separate category.
Does deleting a message delete it for the other person?
No. Apple says deleting messages only affects your iPhone and other Apple devices signed in to the same Apple Account, and you can’t delete messages for anyone else (Apple: Delete messages and attachments in Messages, iOS 27 edition).
I use Messages in iCloud. Is that different?
Yes, in one way. If you use Messages in iCloud, deleting a message or conversation on your iPhone deletes it from all your devices that have Messages in iCloud turned on, per the same Apple page. Check that before a big clean-up.
Warning about Keep Messages
If you choose anything other than Forever, your conversations, including all attachments, are removed automatically after that time. Save anything you need first. The menu names above are from Apple’s iOS 27 guide, and older iOS versions can word them slightly differently.