I once asked the same question about using small format NiMH in a car, but I didn't get a good answer (it was basically "that won't work"). So I'm not sure on the answer to that question.
As for scaling up Prius hybrid batteries, I was told that they are made to be power-dense (as you would expect when you have a very small battery trying to move a car), but a good EV battery has to be much more energy-dense to avoid adding too much weight. I've never seen any numbers behind that, but it sounds plausible. As I understand it, the main advantage of lithium over NiMH is that lithium is notably more energy dense, but even after a decade of improvements lithium batteries are still pretty darn heavy.
Trying to get back on topic...before the RAV4, I converted my wife's Prius to a PHEV. I'd been looking for ways to use less gas for years, and the Prius served our needs well, so I was pretty excited about the conversion. I thought it would be perfect. I loved it for three days...then I hated it.
I realized that I loved when it was working on battery, and hated everything - the noise, the sluggishness, the cost, the pollution, the support for OPEC - that the gas engine did. I was annoyed every time the engine came on, and it came on a LOT. That was when I got serious about buying a BEV (there were none for sale at the time; the Roadster wasn't even available yet) and not buying gas again. I finally managed to buy the 6-year-old RAV4 from an online auction, and had it shipped to Seattle from Utah. When it arrived, it was the first BEV I had ever seen.
I wasn't sure what to expect when the RAV4 first arrived...but it worked out (so well we ordered a Roadster soon after) and indeed I do not miss gas at all!