Interesting. But you should also include a cultural/sociological component. The Roman example actually illustrates a social disposition toward empire building. Alien cultures demonstrating hightened "empire building" periods would likely be expansionist even if their technology matched their conquests. The development and exploitation of high tech weaponry would give non-expansionist cultures the ability to expand.

So, talking through this, looking at the Roman model again -- Rome's technology matched its neighbors during its all important expansionist stages (at least its military technology did, for the most part -- they may have had better spears, armor, and swords, in some cases, but their foes had them too) ... societies/cultures begin nation building once the right social pressures are in place (i.e. ROME -- population pressures; the need for young males to join the army, acquire financial rewards, land, and prestige within their culture so they could grow to be the next generation of leaders of the nation). The advent of gunpowder or other destructive technologies seem to have allowed more rapid expansion of some nations and/or allowed one nation to subjugate another of equal size (American frontier expansion in the face of Native American resistance). In this example, the weapons and technology gave the edge to the US settlers. 

Technology itself doesn't necessarily indicate an expanisonist phase would follow. A sufficiently advanced technology (weapon OR defense) may make encourage pacifist nations expand and conquer a world or more.