iMessage using lots mb of ram.
I've noticed iMessage is using up 619mb on my iPhone. Why is this happening and how can I lower that to 0mb?
are you seeing 619mb when you check your storage on your phone? if so, that's the physical footprint messages are leaving on your phone.
the reason why messages can get so big is because of MMS, so if you're getting pictures and videos, those are saved in messages and not moved to your photo library. if that's the case and you're attached to them, save them, then delete the MMS from your messages. this would free up space on your phone.
also be sure that if you are using icloud photo library that you have the pictures set to Optimize iPhone Storage so the full resolution images are stored in icloud as opposed to taking up unnecessary space on your phone.