Essentially, when using a source build, you are locally compiling the whole engine from source files as well as your project, when you use the launcher version of the engine you are downloading a pre-compiled version of the engine and only your project itself will be compiled locally, there is unfortunately no way to avoid the larger output size.
Is there a specific reason you need to use a source build of the engine? Depending on what you’re using it for (building projects that have server targets as an example), you can actually create your own Installed Build of the engine (the same as what is provided on the launcher) using BuildGraph that includes those features (and functions the same as the Epic provided binary build) with much lower built project sizes.