Bulldozer Question

[Paul Ramsden]

I know its not really a CMP question but you blokes seem pretty knowledgable about these things.
In have been reading accounts of the British using D4 and D7 Bulldozers. What do these designations actually mean? I presume it is weight or something. Picture comparisons or dimensions would be useful.
The reason I ask is I read an account of the British parachuting D4s into the Borneo Jungle and as a model maker thought it might make an interesting diorama. However in 1/35 scale I can only find the resicst D7. Would it be feasable to parachute a D7 or are they way too big?
I also found a Clarke Airborne Bulldozer, how does this relate in terms of D rating?
Thanks
Paul