A unitless model's drawing data units will be millimeters (I believe).
It will not scale when inserted into a new drawing because it is defined as unitless.
Treating it as if it was in mms, you need to scale it up by 25.4.
You should also open it in the block editor and change the units in PROPERTIES to INCHES.
In that way in the future it will scale appropriately upon insertion into another drawing,
assuming that the new drawing has the insertion units and target units set properly.