diff --git a/cmd/mountlib/mount.go b/cmd/mountlib/mount.go
index a8a467522..0a75fe302 100644
--- a/cmd/mountlib/mount.go
+++ b/cmd/mountlib/mount.go
@@ -201,6 +201,13 @@ When running in background mode the user will have to stop the mount manually:
 The umount operation can fail, for example when the mountpoint is busy.
 When that happens, it is the user's responsibility to stop the mount manually.
 
+The size of the mounted file system will be set according to information retrieved
+from the remote, the same as returned by the [rclone about](https://rclone.org/commands/rclone_about/)
+command. Remotes with unlimited storage may report the used size only,
+then an additional 1PB of free space is assumed. If the remote does not
+[support](https://rclone.org/overview/#optional-features) the about feature
+at all, then 1PB is set as both the total and the free size.
+
 **Note**: As of ` + "`rclone` 1.52.2, `rclone mount`" + ` now requires Go version 1.13
 or newer on some platforms depending on the underlying FUSE library in use.