I have had experiences that contradict this! At the same company there was a ~30kloc frontend built by one developer and a ~20kloc backend built by four develop

I have had experiences that contradict this! At the same company there was a ~30kloc frontend built by one developer and a ~20kloc backend built by four developers and the second was much harder to maintain and improve. Multiplying developers on a project can enforce some rigor and extensibility, or it can simply multiply the amount of time and effort available to just hack the next feature in. A single developer can create a mess only they can understand, or they can end up self-enforcing rigor and extensibility in order to handle the scale of the project. (Multiple devs certainly increases the oversight so you’re more likely to know if a mess is being made, so there’s something to be said for this approach).