Not much can be done to decrease the size of the export template binaries, they're large because they're compiled from millions of lines of code, coming from Godot itself and dozens of third-party libraries used by Godot.
Remember that your game distribution should always be done in a compressed form (such as a .zip archive, a Windows installer, or Steam, for example). .pck files aren't compressed, which means that putting them in a .zip archive will greatly decrease their size on disk, especially when they contain many uncompressed resources (such as .wav sound samples).