--buffer-size not working on mount?

I want to clone my vps disk image directly to google drive using clonezilla.

So i do rclone mount --buffer-size=1G gdrive:images /home/partimag

and in clonezilla options i use split files to 500MB

but when it copies, rclone says (with -vv) that there are some small byte numbers written every few seconds.
Why rclone does not first cache those clonezilla writes in 500MB parts into 1G space buffer and then do big chunk write?

unless I’m mistaken the buffer is only on reads, not writes.