Assuming that self-driving car tech gets to the point where it's viable as a business, why _wouldn't_ you assume that it'd be huge? Trucking alone is a massive ~$700 billion/year industry in the US (https://www.selectusa.gov/logistics-and-transportation-indus...). Even if you gain only a few % points of that for autonomous vehicles you're sitting on top of billions in revenue. The auto industry adds another ~$600 billion/yr on top of that. Sounds pretty big to me, and there's lots of room to grow from there.
Now, as for monopolies in the market, I don't buy it quite as easily, but I can see it due to the usual "rich get richer" effect. Not to mention training data -- any self-driving company that starts to serve more rides will be accumulating more training data, allowing them to improve their product more and making it that much more difficult for second-in-lines. Look at Wal-Mart, where competition requires big infrastructure, they're bigger in market cap than their closest 5-10 competitors put together.