BMW is very coy about dimensions in its brochures. I was undecided about X1 (F48) vs X3 and asked BMW for the dimensions. They gave me all sorts of runarounds. Eventually one of the call centre guys referred me to a BMW comparator web site which compares BMWs (not other cars). Guess what? The X1 is slightly larger in passenger space than the X3, except for the rear headroom and the load area where the X3 is bigger. Checkout
http://range-comparator.bmwlaunchpad.co.uk/