The size of the ship was doubled from the first pilot episode to the production episodes. They modified the miniatures buy adding a second row of windows around the saucer edge; thus making it appear as two decks instead of one (since the pilot version only had one row of windows around the perimeter of the saucer). Also shortening up the bridge was done to be more in scale with a ship twice the original proposed size.
I´m not sure about that. While it´s true, that the first model had only one row of windows in the rim of the saucer, that row was at the same relative height as in the 11-foot production model. Likewise the relative size of the windows stayed basically the same, as did the placement and spacing of window rows in the secondary hull.
Thus it seems unlikely IMHO, that the "Kirk-Enterprise" was intended to be twice the size of the "Pike-Enterprise".