Two views on this:
1) Roll out Azimov's Foundation again. In brief, the Galactic Empire is huge but corrupt. Their ships are huge but full of out-dated, inefficient, massive technology. On the other hand The Foundation is technologically superior but mineral-poor and builds very small ships packed with highly efficient equipment.
It's worth pointing out that during their first encounters the Foundation ships can't do much more than shake their fists at the Empire behemoths.
2) To build a huge spaceship on Earth and launch it would require a massive technological leap - we simply don't have the power to get something of such mass (even an SE4 escort is twice the mass of American supercarriers) into orbit. The obvious solution is to build segments on the ground, shuttle them up and build the ship in orbit.
This has its limits - I can imagine small ships being constructed from small components that have been built on the planet and shipped up, but can you imagine trying to assemble a dreadnought's power plant in orbit? It makes sense that these problems can be gradually overcome with better technology (which links back to the miniturisation inherent in point 1).
I guess it depends partly on what level of starting tech you're going to assume.
Talenn - I like the sound of what you've said. Is your armour mount system anything like this:
I worked out some figures for armour mounts that are based on surface area*, not mass (assuming mass is proportional to volume), so that big ships can have thick armour whilst small ships have to surrender a lot of space just to have weak armour. I've not worked out how to integrate shields (which would surely operate under the same principle) into this scheme yet...
* smaller objects have a higher ratio of surface area to volume, which means any armour is going to take up a higher proportion of the object.