ROMs are copyrighted files.
Changing the content doesn't make it legal to share, or else everyone would just need to modify a bit in a MP3 and movies and games and all files which are monitored by publishing companies to make then safe to distribute
You need to create a patch file.
The patch contains informations on where it need to be modified and by which data.
It doesn't include any data from the developers, only your own and free data (assuming you used only things you did yourself).
a 15MB patch?
You sure changed a lot of content.
You hacked the game that much without knowing that hacks should be released in patch forms?
Maybe it's just a redesign using level editor tools.