The sizes of the Gidget tables refer to the opening, not the table itself. Both the Gidget I and II tables are the same size overall. If you have a larger machine, you need the II. For example, my Juki 2010Q will not fit into the Gidget I table that I already own... the machine is too long. So I use the Juki on top of the table and adjust my chair instead. However, my Singer machine fits into the opening on the Gidget I, so I can use that machine dropped into the opening with the surface flush.
I hope all of that made sense!
Oh, and one more thing. The Gidget II table has wheels on one side which supposedly make it easier to move when you close it up. The Gidget I does not have wheels.