[MENTION=481085]L0TT0[/MENTION] is likely using a plug designed specifically for 1 1/8" steerers.
Plugs for 1" steerer are available. A good shop will have a selection of plugs to try.
Expansion plugs cannot be "Whittled" down. They have a knurled surface to grip the inside diameter of the steerer and any attempt to file or machine it will ruin it.
-Tim-