In my opinion:
X5 = Small SUV
X3 = Crossover
X1 = Tall station wagon. EPA doesn't classify it as an SUV or crossover at all, but a midsize car, and I tend to agree.
I.e. neither is particularly big. I parked my old SUV next to an X5 just the other day, and was thinking "it's really that small?"
Anyone looking for a people carrier might want to avoid the BMW X series, but especially the X1 and older X3.