Essentially, every planet now is essentially identical except with some modifiers on them that make them more or less habitable, with accompanying flavour text. Habitability only really effects population growth and the overheads of running the colony (200% habitability = 200% running costs). However, none of these modifiers prevent some very silly occurrences, such as Barren, irradiated planets apparently being capable of housing populations of hundreds of millions without issue, despite the obvious logistical difficulties of running a colony where the environment is described as being "extremely hazardous to even hardened robotics" and people needing to wear bulky hazmat suits to go outside, if ever.
Hence, what I'm suggesting is that planets have a semi-hidden "Capacity" factor which indicates an absolute maximum size that a colony can reach. Any planet that is Habitable automatically has a fairly high capacity, and any planet that needs extensive works to maintain life (huge, underground facilities, floating sky-habitats, etc) generally doesn't. Larger Planets have a higher innate capacity, and smaller planets have a lower innate capacity. Modifiers which have obvious effect have some effect on capacity too. High gravity, for instance, is going to reduce it due to buildings needing to be built lower and taking up more useful land area, Extreme tectonics is going to reduce it by requiring earthquake proof buildings, while similarly, ones like low gravity will increase the capacity. New modifiers could additionally be added which could have combination effects, for example "Deep caves" or similar that increase the planet hazard, but increase it's capacity by a similar amount due to the increased living space. Additionally, a "Colony Expansion" repeatable building could be added that fills a building slot on a colony which increases capacity by some amount, but has operating costs, representing the costs associated with running a giant eco dome on what could be the equivalent of Venus.
Why is this a good idea? Three reasons, Firstly, planet size or even really the type of planet matters very little when setting up a colony, other than it's operating costs, which generally become irrelevant as the colony gets bigger than 5 or 6 and has enough useful exports to fund itself, and becomes even less relevant once you are looting AI cores left right and centre to cut the operating costs by half. By adding a capacity factor, more care needs to be taken about where you situate your main colonies, and what to use as just simply "mining colonies" or similar for collecting or storing the useful resources you need. You can't simply out grow the operating costs or grow the colony to a point where you're dominating the market anyway if the colony physically can't grow that much.
Secondly, for thematic purposes it feels more natural that colonies be generally situated on planets actually capable of sustaining life, and anything that isn't on a habitable planet is more out of necessity to obtain some kind of material reward for doing so (eg, resource extraction, tech mining, etc).
Thirdly: Flavour. Good quality habitable planets should feel like Gleaming Jewels in a Sea of filth, and even if they don't necessarily have amazing Primary industry stats on them, should still be valuable from the livability and maximum population that can be put to work on secondary industries such as light industry, refining or shipbuilding, even if it's importing everything from your smaller mining colonies that can't maintain a population of more than 4 or 5.
