Phone icloud says items in storage are many times larger than they actually are

Several times per year I get notifications saying that my icloud storage is 90% full and services are going to be cut off soon. Every single time, I open up icloud account settings and find that something is listed as consuming many times more space than it actually consumes. For example, today I open settings and find a bar graph that shows that Messages is consuming 3.8/5.0GB, but when I click on Messages I see that all of my messages consume only <750MB. Phone backups consume 665MB. The third largest consumer is 12MB. So once again, I have to figure out why Messages is listed as consuming 5.5x as much space as it actually is. I guess I'll just unsync/resync it with icloud yet again, because this problem has been occurring regularly for years and Apple is never going to fix it.