Generally speaking, you should avoid creating a big source file. Break down into smaller files / modules is more readable and reusable. But that may not true for QML, due to signal propagation across several components: with more files, you may need to write extra code to link them. And that will cause trouble when refactoring.
In this article, I will explain the problem of signal propagation and propose an Action-Dispatcher pattern to simplify QML application architecture. It is inspired by the Facebook Flux application framework.