iPhone Storage/space issue

Hello everyone!

So I've been looking through my iPhone 6's usage and stuff, then I noticed that it says my Photos is taking up like 70GB or something like that. Then when I tap into it, it shows: "Photo Library: 58.2GB", "Synced from iTunes Library: 12.4GB", and two other things that are very insignificant. You may think that's just because I have tons of photos on my phone, but on iTunes it says that "Photos" only takes up about like 8.03GB whereas "Other" takes up about 73.54GB. I was wondering if anyone can help me figure this out and why it's doing this. My iPhone 6 is obviously a 128GB and for some reason the photos is taking up lots of space. Oh and another thing...I was looking at another thing and I wasn't sure what it'll do so I want to ask you all for some help on this. So I went into Settings -> Photos & Camera and when I try to turn on "iCloud Photo Library" it asks if I want to delete the 20,000 photos on my phone when the Photo app only shows 10,000 photos. I have no clue where the extra 10,000 photos are and it also asks me if I want to delete them and something about syncing from iTunes is not supported. I wasn't sure if that'll delete the photos from my iMac or not. Just a last question...whenever I try to back up my phone, it keeps saying that my phone is disconnected, but the phone is connected to the iMac directly via the lightning cable. I have no clue why that happens. Sorry for the long post, but I hope you all smartypants out there can help me out. 🙂 THANKS!

-CIownZ