When a country converts to metric it does not simply change the dimensions from inches to millimetres, but takes the chance to make the range offered more rational in both terms of international usage and what already exists. So some things change a lot, while others do not change much.
Tubing has remained in inch sizes, but it is described in metrics. Tubing is specified by its outside diameter and wall thickness. The main sizes used for the legs of trolleys to which castors are fitted are shown in this table.