To be sure, there are a number of major states that do not register voters by party, such as Texas, Georgia, Washington, and the keystones of the industrial Midwest: Illinois, Indiana, Michigan, Ohio, and Wisconsin. If they did register by party, Texas, Georgia, and Indiana would almost certainly add to the Republican total; the industrial states probably less so.
Comparing party registration totals between states at a particular point in time can be a bit misleading, particularly because some may have recently completed a purge of “dead wood” on their rolls while others have not. Often, the trend line over a series of years is more valuable to look at than a one-time registration count.