Deleting iMessage conversation while not removing large attachments on iPhone

i was running out of storage on my phone and my settings app told me that it was bc of a 20-something gb text conversation, so i deleted it. now it’s showing that deleting it not only did nothing but also means that i have to delete every ‘large attachment’ that was in that conversation individually. i don’t even know what to ask here, shouldn’t deleting the conversation have gotten rid of the photos and videos? if not then what the **** was the point of me deleting the conversation???

[Re-Titled by Moderator]

Original Title: imessage & storage