Well, there are four counties in England bearing the name "Yorkshire" - North Yorkshire, West Yorkshire, East Riding of Yorkshire, and South Yorkshire. All are part of the greater region "Yorkshire and the Humber." When you look at England by county, only North Yorkshire of those three is exceptionally large. I assume you mean the historic county of Yorkshire, though, in which case the answer seems to be simply that it is a successor to the Scandinavian Kingdom of Jorvik and shares largely the same territory.

In any case, the present counties of Yorkshire got their current borders from the "Local Government Act of 1972". This act was intended to make local government and administration more efficient.