Q: Photos

My iPhone 6+ 64Gb is only a few months old and I only had a couple of hundred photos on it. I back them up to Photos on my MacBook Pro whenever I have taken new ones, but tend to leave them on the phone. I've recently noticed that all Photos from my MacBook back up have turned up on my iPhone, meaning I have over 12000 on my phone. How can I remove them from the phone? if I try to delete them from the phone I get a message telling me it will remove them from Photos on my MacBook. I've done a test by removing a duplicate from my phone, which subsequently disappeared from my MacBook Photos. My concern is if I delete them from the phone, I may lose all those on the MacBook. So, how did all 12000 end up on my phone in the first place and how do I free up space on my phone without losing all my photos?

Help appreciated!!!