Messages Taking Up ICloud Storage

My messages take up 100 gb out of my 200 gb of storage. I dont have enough money to upgrade. For some reason, my phone randomly retrieved all of my old messages from years ago that I deleted. Previously, I only had them available on my mac (for some reason). I tried unsyncing my mac messages to see if that was the case, and nothing. I also deleted all my messages older than 30 days on my phone, and the messages storage increased. I'm thinking maybe it's because of my computer, but I don't know how delete all of the messages. Any help? Thanks in advance.