UE4 will only package the following assets:
- Assets referenced directly or indirectly by any of the maps included in the package.
- Assets referenced directly or indirectly by the classes set in your project setting’s “maps and modes”.
- Assets in folders you manually specify in the project packaging settings.
If you don’t specify a list of maps to pack in the project’s packaging settings, UE4 will package all maps it can find by default (since maps are often loaded by name, there’s no way for UE4 to know which ones are actually used by your game). The Starter Content contains a few maps, so and those will be included by default, along with all assets they reference (aka: almost all of the starter content). The same goes for many assets from the Marketplace: many of them contain demo maps.
So you either follow IronicParadox’s suggestion and migrate your maps to a different (clean) project or go into project settings and fill the list of maps to package, which will keep the unused assets in your project but won’t package them.