[Alneyan]

Try to design a Destroyer, and then load it up with population. You will see that it cannot work with your basic technology, since you will not have enough space to put one million settlers on it.

I will develop my explanation of the "why you must use Starliners to transport population" (at least in the early game).
- The Starliner hull gives you 1798 cargo space (if memory serves), which is enough to transport one million settlers (requiring 1,000 cargo space).
- The Starliner module gives you 101 cargo space, which makes 202 if you put two of them on your Starliner. It allows you to reach the 2,000 cargo mark, and so you can transport two million settlers with such a design.

If you do not need the extra storage (meaning you will transport units), you could put life support or something else on the Starliners instead. Otherwise, you will need to cope with them until you can develop higher hulls with the best cargo technology, since you will need 10 Cargo Bays III to transport one million settlers.

Incidentally, there is no need to put three Starliner modules on the basic Starliner hull, since the modules on themselves offer little advantage. Instead, put the three modules on your Large Starliners, allowing you to ship three million settlers with every travel, since this hull gives you 2697 basic storage ability. (Plus 101 per Starliner module, and so the 3,000 needed to carry three millions)

In short, you only need to have a cargo space of 1,000 per million settlers, and Starliners are the only way to achieve this until you can reach much higher levels of technology. The Modules themselves are merely helpful to reach this mark, and gives you all the Life Supports you need as well.