The Imperium does have a disadvantage in technology based purely on its own size, mind. Logistics, especially in a universe where FTL communication is less than guaranteed, has always been their primary limiting factor.

The lasgun is an excellent general example of this. It is not the most powerful gun the Imperium can make, or the most accurate, or the most efficient, or even the smallest. What it is, however, is easy to mass produce, robust enough to stand up to a ridiculous degree of punishment, and capable of being recharged at a wide range of power sources. Hell, throwing the clips into a fire will do in a pinch. When you consider how many wars the Imperium has to fight at any one time, and how many soldiers it has to keep equipped, such virtues become worth infinitely more than the greater hitting power of a Pulse Rifle.

Eventually, the Tau are going to expand beyond their relatively dense little cluster of star systems. And then they are going to run into all the problems that the Imperium faces, and be forced to adopt a sustainable baseline of tech that they can work their Civilisation off of. Heck, until they develop faster warp travel, they are physically incapable of forming a unified empire even a fraction of the size of the Imperium.