[jc173]

I've noticed this problem too with some of the TDM races. They have designs for different tonnage ships, but for some reason seem to always build the largest possible. In particular I noticed that once they have dreadnaught tech, the Rage always builds heavy attack ships and completely forgoes the light attack ship design. It might be something really odd and subtle, like the amount of resources they have or the production rates of their shipyards. On the other hand I've seen other races with the different weight class ships still put out smaller ones late in the game. Unfortunately your guess is as good as mine.