Aerodynamics seem to be a more critical factor for electric vehicles. You want the vehicle to be able to cruise while using as little energy as possible, and weight doesn't matter as much for maintaining speed.
It's the same reason that most modern trains are electric or diesel-electric rather than pure diesel.
Many aircraft tow tractors are also electric these days, and they haul some fairly heavy loads.
The Model X will be ~10% heavier and Musk has stated recently that the range is expected to be ~10% less than that of the Model S.
In other words, obviously a heavier vehicle will use more power than a lighter vehicle, that's true with any fuel source, but it's a small difference in this case. They could put a battery pack with 15% more capacity in it and the range would be about the same as the Model S, but they're probably going to use the same battery as the Model S for compatibility and manufacturing reasons.
The energy required to maintain a given speed is almost independent of weight. Most energy loss at speed is from wind resistance. Rolling resistance will be affected some by weight, but it's not the dominant factor at speed.
More initial weight means more batteries needed in the first places, which means more weight and even more batteries because of that.
Reducing weight on electric cars is really beneficial:
Less weight => less batteries => less weight => less batteries...