iCloud storage/iPhone storage discrepancy?

I'm experimenting with iCloud storage (the free 5g) to save space on my iPhone. It's not working for me because I want to get the photos into Photos, but I can't sync my Photos library with iCloud because it has years' worth of photos and videos. So I decided to download my photos back to my iPhone so I can move them into my Photos library via syncing to the Photos app. I have 10 gigs free on my iPhone, and my iCloud library has 4.8 gigs of photos in it. But when I change my settings on my iPhone to download the photos, iPhone says I don't have room to download them. Any idea whats up with that?