It's not according to the research order, because the AI builds scads of Bulk Transports all through the game. My guess is it's hardcoded into the game to build --% Heavy Cap Ships, --% Transports, and --% Starfighters. Then it keeps up with how many of each it has built and builds what is lacking when it has the resources. This is entirely a theory, I've never seen anything on this...but it would explain why there are so few Heavy Cap Ships built and so many of those crappy transports!