Didn't find a smiliar post. Anyway, playing TOA I upgraded the default Terran colonizer with new engines and removed one of the two colony modules. New ships when launched are not asked to select how many colonists to carry, and when arriving at planet will say "no colony module" present.
Even the default colonizer will carry max of 250 colonists. Aren't the modules each capable of carrying 250? And it's got two clearly.
Maybe due to attrition half die enroute...