Alteran, those pictures are amazing, and more than prove the point. Your sentiments regarding the jumbos being the 5" models blown up in size also nails it.
I'll tell you the main reason I don't like the jumbo-D: It's based off of the 'These Are The Voyages...' CG version, which is as bad in my opinion as the 'Valiant' version of the Defiant.
To further Razorgeist's comment, I'd love to see a jumbo-Grissom added to that list of non-hero ships!
One thing that Ben mentioned ages ago, was the possibility of a D with saucer separation. I don't know what became of that idea, but I'd certainly be down for a larger-sized Prometheus that could split into three (or be held together magnetically).