You have to ignore quite a bit for the 'realism' theory to work re. G1 combiner sizes. Defensor has a patrol car and a patrol bike as legs of roughly equal size, and arms made from a helicopter and an ambulance which are both also of equal size. Bruticus has tank and jeep legs the same size, and helicopter and space shuttle arms the same size. And they're all roughly the same size as each other.
AFAIK the Predacons are usually depicted as Transformer-scaled animals, kind of like the Dinobots are way larger than their real-life equivalent counterparts were.