The 135 is a development of the Boeing 367-80 (which Tex Johnstone famously rolled with the press on board), which also gave rise to the larger Boeing 707 - itself the basis of the E-3 AWACS. Physically, it's a bit bigger than a Boeing 737-800 but quite a bit heavier at 135 tonnes. The Voyager is around 60ft wider and longer and 100 tonnes heavier.