Reference attributes in inline-editing in DataGrids are currently handled using the standard DataSource library object, setup without paging parameters. The default page size in that case becomes 10.
Currently there is no way to influence this.
This is definitely not the desired behaviour, though some (arbitrary) limit is desired for drop-down element. Reference drop-downs generally default to a maximum of 250 items (with a warning on the development console if the size exceeds this number) because fetching lists that are too large and attempting to set them in drop down select boxes renders some browsers (primarily IE) unresponsive and makes it hard to track down what is causing the hang-up.
If you file a bug report in MXDN it will be picked up.