[F2]

The Model X is most certainly an SUV from a size perspective.

You're confusing its shape for its size. Likely it's slopped rear hatch vs the vertical rear hatch of that gives the box shape you associate with SUVs.

For the record SUV stands for Sport Utility Vehicle and thought there's not really one set of characteristics or definition for them, their utility is typically the thing that defines them. This might include things like passenger and cargo capacity, off road capability like ground clearance and 4wd, on road capabilities like towing capacity, etc.