Unreal by default puts in the packaged product all the maps included in the project (and no one usually wants this, this is why Whitelisting maps exists).
You shouldn’t care too much about your sample map being included in the final build, it’s not something you can control.
If you want to release a lightweight plugin and provide the user a big example project (500+ MB), you should upload it to a cloud storage and link it in the marketplace and in the product documentation, so the user can choose weather or not to clutter his drive