Transports and colonizers are big ships - 300 kt of space, that you get
before you research ship construction twice and get destroyers which are also 300 kt.
They have a built-in downside, they must have fewer engines and must carry certain components. The transport must be 50% storage bays. The sattelite bay counts as a storage bay, so it's great for putting sattelites on warp points or other spots you don't have a planet.
Using an escort for that purpose, with it's ability to have 6 engines, doesn't really seem to be a better choice
