In regards to your first question above about the size of the dozer in comparison to the truck, I found this video of a 777 being pushed up a hill by a D11. Your scale looks to be pretty close to me. What do you think?
http://www.youtube.com/watch?v=RMtaJsWRlMc
If it were me I would want to size the shovel where it somewhat evens out the time spent operating the shovel as compared to driving. Like Joe said above, digging is more fun than driving. I have no experience operating the models, but I know in the real world I would much rather operate a hoe, than drive the dump truck. At the same time I wouldn't want it to take 100 scoops to load the truck as that would get old too. Looking at a 777 with a mini ex would be pretty intimidating