They are about the same size, GTB 4-Cell (non spectrum edition) is a hair smaller.
XBR: Footprint 1.54” x 1.18” (39 x 30 mm)
GTB: Footprint: 1.49 x 1.16" (37.8 x 29.5 mm)

The GTB is lighter because it does not have a heat sink.
GTB: 1.01 oz. (28.5 g) w/o wires
XBR: 1.49 oz. (42.2 g) w/o wires

The GTB has better on resistance.
GTB: On-Resistance: 0.00040 ohms (brushless), 0.00013 ohms (brushed)
XBR: On-Resistance: 0.00120 ohms (brushless), 0.00027 ohms (brushed)

GTB can handle faster motors: 3.5T vs 8.5T

XBR has automatic Lipo cutoff, GTB 4-cell (non spectrum) does not.

XBR has auto brushed/brushless motor detection, GTB does not.