If you include the StarterContent in your project, it will bloat the content a lot. Currently, when you Package a build, it will put ALL content from your project into the .obb. We are planning to allow for selecting only some maps when you package, which will then only put the assets required for those maps into the .obb.
If you have code tho, and you load assets by string name, you will have to do some extra work to tell the cooker program about those assets.
Anyway, either start over with a clean project, or go into your Content directory and start deleting 
BTW, the .obb file is actually just a renamed .pak file (UE4’s “virtual file system”, which is a bunch of jammed together files).
You can use UnrealPak tool (in Engine\Binaries\Win64) to list the contents:
UnrealPak -test
Hope this helps!
Josh