Why does my Messages take up so much storage on iCloud compared to my local storage?

The only device in my Apple Account is my iPhone, and locally my Messages only takes up 457 MB, but in iCloud my Messages take up 3,2 GB. The option to offload messages to iCloud to preserve local storage is turned off.

My questions:

1. Why does my Messages take up more storage on iCloud than on my iPhone, even though it's the only device?
2. If I under "Messages in iCloud" settings change the "Keep Messages - Forever" to a shorter period of time (like 1 year), will the messages also be kept for a shorter period of time on my iPhone, or is this a separate setting from the iPhone's local setting to keep messages stored forever?

Messages locally only takes up less than 0,5 GB.

Messages in iCloud takes up 3,2 GB, even though my iPhone is the only device in my account.