Given the limited fuel range of N911, I would wonder why the fuel tank wasn't expanded
From the Shuttle Ferry Flight blog:
...the SCA has a very limited range when ferrying the Orbiter. That range depends primarily on the weight of the Orbiter and the air temperature. Winds are also a factor. The heavier the Orbiter is, the less fuel we can load into the SCA, pure and simple.
This despite @RusselBorogrove's note that the Shuttle dry weight is below the the 747-100's payload capacity.
The Shuttle was above its dry weight during (at least some) ferry flights. After landing, they did not remove all remaining cargo, empty the fuel tanks etc. before it was ferried back.
The SCA was also modified: its fuselage was strengthened, increasing its weight and reducing its payload.
They were close enough to the weight limit that adding more fuel wasn't an option. In fact, they couldn't even use all the standard fuel tanks.
In the words of one of the flight engineers on the SCA:
There are four main, one center wing, and two reserve tanks. We normally only use fuel out of the mains and reserves. We don’t use fuel out of center wing mainly because the Orbiter just takes up too much weight. The airplane can only weigh 710,000 pounds at takeoff. So with a 200-and-something-thousand-pound Orbiter we can’t put a full load of fuel on. So we never get fuel in the center because you make it too heavy.
Finally, places like Edwards are at higher altitudes which decreases performance.