The idea goes sort of like this; It is canon to the show that, in Irken society, the taller you are, the higher your social status. Those in command do not just happen to be the tallest Irkens, that is actually a prerequisite for the job, in fact, the ONLY one. The only reason there are two of them is that they both happen to be EXACTLY the same height (again, show canon). That's why I implied the Tallest presumably do NOT have any body modifications. If they were allowed to have such modifications one would expect them to be in a perpetual cybernetic arms race to each become taller than the other, since they could then rule alone.
So the idea is that, if Vexus want's the Irkens to serve her, she may have modified her body to play into their culture and exploit it. Her forces are probably to limited to maintain such control over the Irken's long term by force, and she is presumably WELL versed in alternate ways of keeping power (you don't get to rule as queen for almost 2000 years if you aren't). Because of the way the Irken's think, if she looked a bit like the Tallest, but even TALLER than them, then, when she claims the throne, it might actually legitimize her reign in their eyes. Or it could be as Higurashi suggested, she could be going for strict disguise. We KNOW she can body morph, but that her morphing ability has limits.
"For I have heard the Song of War, and to me, it sounds just like "Please, Mister Custer.""
Cogito Ergot Sum, "I think there is something wrong with this rye bread." (Aplogies to Seafrol.)