Does this feel backwards to anyone else? States with small populations have inflated sizes. States with large populations are tiny and thus look insignificant.
Um, I think you may not be a native english speaker, so just a semantic note here:
inhabit breaks the "in" prefix convention (or the "in" is not a prefix) in that the meaning of "inhabit" is "to live there". So an "inhabitable place" is on where people could live. If you mean "a place where people can't live", the correct term is "uninhabitable".
Note - the word "habitable" means the same thing as "inhabitable".
It is just one of those confusing English language things, where all rules are more "guidelines" than hard rules, and there are exceptions to the exceptions.
Another word that follows a similar pattern: flammable and inflammable mean the same thing. Namely "easy to burn or start on fire".
So I think that to support the empty statement in your first sentence, your second sentence should read:
Japan - 4/5 of territory is uninhabitable. China - the same. Russia - Siberia is almost uninhabited.
edit - The statement "Siberia is almost inhabited" is also a funny/sarcastic way of saying "Siberia is almost uninhabited".
As a non-native English speaker, your semantics are incorrect. There is a nuance.
Habitable is an adjective and means it supports life (usually of humans).
Inhabitable is an adjective form of the verb "to inhabit" and deals with one's intention.
So habitable means something supports life and inhabitable means intending to live there is viable, possibly with some modifications to the environment to make it habitable.
As a native speaker, I'm going to push back on the nuance you suggest, and say that that distinction certainly does not exist in my idiolect. Which jibes with a quick dictionary check.
Neither word connotes intention, as far as I can tell.
Related: "flammable" and "inflammable". Either means that the object in question can be set on fire. The second word is an abomination that should never, ever, ever appear on warning labels. However minor an abomination it may be, it is an abomination all the same.