Just because you're not in the largest market doesn't mean you can't be successful. You just have to look at what "successful" means to your company. Is it simply turning a profit, or is it seeing sustained revenue and profit growth over time? A stand on the side of the road that produces 50 cups of $.05 lemonade a day and sells them for $.25 a cup is successful for a 10 year-old, but obviously not for Minute Maid. It all depends on how much profit Isuzu desires to make.
Not being in the largest market in the world means that they don't have the tremendous footprint and expenses associated with doing so, namely property ownership for dealerships and plants, salary and fringe packages for employees manufacturing cars, etc. In theory that means they can sell substantially less vehicles each year and still make the same amount of profit as GM, so long as they keep their expenses lower per vehicle. Staying in its home and lower expense markets (without US labor unions and export/import duties to weigh its vehicles' price down) maintaining a much lower production cost is very attainable.
I wouldn't say that. Regulation tends to drive innovation, since car manufacturers will never abandon market segments for which there is still demand. Cylinder deactivation technology, hybrid-drive systems, CVT technology, smaller displacement turbo-charged engines, variable valve technology, and direct injection are all technologies that have come about as a result of the need/desire to squeeze more HP and/or MPG out of existing engines. Nissan's new Juke is a good example of what can be achieved; it's projected to achieve 26-30 MPG in the turbo 4-cylinder / CVT model, which is nearly 7-10 MPG better than any other SUV or cross-over currently made by Nissan or Infiniti.
I suppose you could argue that increased fuel costs and emissions regulations in the 1970s lead to a generation of crappy cars (Mustang II for example), but then again they didn't have the computer technology that we have today. Given no other options, manufacturers simply downsized engine displacement, resulting in significantly less power. But I just can't see that happening again.