You could argue: If it's 1/10th the size, then it's about 1/3 the length and width. So, average distance (if you pick two points uniformly randomly) will also be around 1/3. By this logic, it should be around 1000 EUR.

Not that it changes much, but the BC100 is 4000€ for second class, not 3000. Both is too much to make it really popular, while I'd say Austria's pricing is extremely attractive even if the country is smaller. I pay more than half of that just for public transportation in my city in Germany.

All trains plus many local transit options as well. It's usually valid in most bigger cities. My ex-ex-employer had given me one and I've used it many years. Incredibly liberating when you have the ability to hop onto any train and just sit. Reservations were sometimes problematic because popular trains had little amount of not-reserved free seats but Bahn.Comfort seats usually saved the day.

And Switzerland is also small but way more expensive, but, that's not how things work. That's not how anything works.

Since most public transport is pretty localized the difference should be negligible - you're rarely going to travel a significant distance in relation to the size of the country as that would be unfeasible for a daily commute.

In practice it's not much different from a yearly local transport ticket plus the occasional long distance trip included

I'm not sure I understand that logic -- Germany has far more potential customers than Austria. And it's not like the average German travels ten times as many miles by train as the average Austrian. Also, the BahnCard can only be used for some, but not all local transit options, which looks like a hassle.

1/10th of the size doesn't mean 1/10th of the costs/usage/maintenance/&c

Long journeys are rare, everyday journeys have short distances. A larger area therefore only causes a small jump in costs.

Size is irrelevant.