Sorry, I misunderstood. I thought you were disabling checksum. Are you syncing the contents of a crypt? If so, crypt doesn't store checksums so would default to size unless you explicately enable it (like you did) which would in turn download the file to check its checksum before syncing/
If it isn't a crypt then you can run a rclone check -v to see whats going on for the file(s) in question.