Incorrect iMessage iCloud backup size - 0.7Gb vs 4Gb

Hello,

does anyone know if there is a reason why my iMessage backup is 5x the size of the messages themselves?

On my iPhone, the size of messages is 0.7Gb, after turning on iCloud backup for iMessages, it shows the size on iCloud over 4Gb and it filled the entire storage.

I had the same problem before, that's why I turned off the backup, waited 3 months for the backup to be deleted from iCloud, and with 0.7Gb of messages on iPhone I now turned the backup back on.

Even my "Top Conversations" are only 0.6Gb.

I can't use iCloud at all with this error. Thank you for your help.

Jirka