Bigger means bespoke. More NRE, more paperwork, more one-offs. Smaller means the entire supply chain is simpler and easier. Management is easier. And of course, the more times you do something, the easier and cheaper it gets.
This is very much an unproven assumption. There is nothing in the history of nuclear technology to suggest that what most industries experience, an industrial learning curve that lowers production cost, will be found for these reactors. In fact, just the opposite - the more experience we get with them the more likely we are to find problems that are very expensive to fix. This is even more true now that they've given up on doing serial production in a single factory.
Mass production is a no-brainer.