WebIDE performs a build step which creates that preload files before the deployment.
Those files are a package of all minified single source files, which are used at runtime to speed up the loading of the app, by just requesting one file instead of many.
However I really wonder why the file becomes that big. Of course a preload file with 19mb would take really long to load. Do you have that big source files or is there some random data inside those files which blows them up? Could you please have a look at the content, whether it contains nonsense?