I have a weird conspiracy theory that the reason the Federation colonies and worlds seem so underpopulated when the numbers are given is that the majority of Federation citizens are uplifted code living inside virtual machines or completely solipsistic - living inside of holodeck/food replicator/life support boxes run by perfect fusion reactors stacked like cord-wood somewhere.

It would make sense considering that everyone else who is living outside in the 'real' seems obsessed with pastoral/analog almost Victorian past-times like card games, plays, music recitals, hiking, etc.

A philosophical split occurred at some point, people who want to live in the 'real world' for the majority of their lives and people who bubble off into their own personal universes. Why bother even talking about the latter if you're the former, they have net zero effect on your society and existence.

It also explains why the Klingons and Romulans are such a threat despite having archaic social structures of their own - they simply have more raw 'real' population than the Federation does. Klingons can put troops on the ground, The Federation cannot - which is why we never see anything remotely looking like truly mass scale Starfleet ground forces, they don't have them. The entire Starfleet military doctrine involves space superiority - which is why actual planetside wars are so terrifyingly dire to them, they simply don't have the capacity for it.