Games are software, if you sell 10 copies or 10 million, it’s basically the same amount of work. That is: no matter how big your development budget is, or how many you expect to sell, 100% of that budget goes into developing 1 copy of the game.
However, the actual budget does scale with the number of expected sales. A game that is expected to sell 5M copies is going to have a much larger budget than a game that is expected to sell 10k copies. Add to that the lower price of indie games and there is a huge budget difference.
The end result of this is that as a gamer I can choose between a €20 indie game representing maybe €500k of development effort, or an €80 AAA game that represents €200M of development effort. The AAA game may cost me 4 times as much, but it gives me 400 times the value.