We Have Reserves: Heavily averted thanks to the quality-over-quantity philosophy of the Imperial Legions. Even the most kill-crazy of the Empire’s historical generals (which is to say, the House of Sargas, for the most part) have been very determined indeed that the death should all, so far as is possible, happen on the other side, and very protective of the lives of their own men. Of course, if it’s possible to deceive someone else on the enemy side, or even on a different enemy side, into being your reserves… well, that’s just shiny. (See: Enemy Civil War).
(The people who have noted the disparity in quantity and tried this against the Empire have discovered that they thought of that: that’s what massive orbital bombardments and the Nuclear Option are for. It’s the only way to be sure.)