Open to other suggestions. Also on UI, which I haven't used as much.
I think that before we decide how to tackle the capacity per size issue, we should first decide what the orbital habitat modules should cost. Let's start by checking how much normal infrastructure cost, as so far I haven't seen anyone complaining that it is too expensive.
The infrastructure is very simple to build and costs two duranium per unit, no more no less. On Mars this translates to four hundred duranium per million people and, by extension, to four hundred build points for every million of people. As orbital habitats allow you to build colonies everywhere and they do not require any population for agriculture and life support, one could argue that two thousand BP for a million colonists is reasonable. Currently it costs two hundred BP for fifty thousand people or four thousand BP for a million colonists not counting life support, bridge and armour (it is a ship after all). Overall when I try to build it in aurora, using composite armour technology, I have to spend 6808 BP for a habitat for one million people, or thirty four times as much as I would have to pay for infrastructure on Mars for this many people. By expending orbital habitat module capacity to 250 000 people without changing cost I would have to pay about seven times as much as I would have to pay for infrastructure. That could be more or less considered reasonable.
There is a catch however. You get free infrastructure from civilian sector. A lot of it. Which you can use however you want. In most of my games I would guess no more than one tenth of the infrastructure in play is made by my factories, with all the rest coming from civilians. If that is correct, than even by increasing the orbital habitat module capacity fivefold, I'd still end up paying
seventy times as much for space for one million colonists as I would have to pay if I was using infrastructure. That's a pretty steep price.
To be honest I'm not sure how to deal with the issue. Making orbital habitats cheaper than infrastructure would be just wrong, but the fact you can get so much of it for free form the civilian shipping lines is undeniable. On the other hand the habitats are not supposed to replace normal colonization, so they should be more expensive, but not to such a large degree I think. Overall I'd say the right cost would be one thousand BP for one million people for habitats, but that after including armour, living spaces and such. That would require the current cost to be reduced to twenty BP per fifty thousand people. In most of my games that would mean my factories would have to work for a year to create enough habitats for five million people, which is enough to support a hundred mines or factories, if my memory serves. If so creating a colony on Venus for four hundred mines (as an example) would require eight years or so for both habitats and mines, which seems just right. It is of course only my opinion and all of the numbers here are highly speculative and very much dependent on the play style of the particular person.
One thing about modifying the size of the habitats - they need to be towed to their target location, which means they cannot be too large or making a tug for them will be very difficult.