Surely that can't be right.
The very nature of a small market, implies that the solutions are easier - because of less competition, so less innovation is needed, therefore lower return.
With larger markets, not only do you have to solve a problem - you have to solve a problem better/different than your competitors.
Not all solutions are created equally and not all solutions solve the problem in the way the customer wants.
It's harder to separate the signal from the noise when the customer doesn't know what the solution should look like - and when the competitors don't know either.
Quite often, in a large market, many competitors exist largely because no 'significantly better' alternative exists. Think about all the crappy desktop software (from the 90s or early 2000s) that many small businesses still use.
In a smaller market, you are more likely able to get away with simpler solutions - e.g. a CRUD web app, versus having to come up with a more sophisticated solution because competitors have already squeezed as much out of CRUD apps as they can.