Photos on iCloud, but still taking up tons of space!

One possibility -- note that the photos are only optimized when the space is needed. See the following from Set up and use iCloud Photos - Apple Support

If you turn on Optimize Storage, iCloud Photos automatically manages the size of your library on your device. Your original photos and videos are stored in iCloud and space-saving versions are kept on your device. Your library is optimized only when you need space, starting with the photos and videos you access least. You can download the original photos and videos over Wi-Fi or cellular when you need them. You can use Optimize Storage on your iPhone, iPad, iPod touch, and Mac.

Optimize is an "as needed" option. If you are not needing the storage savings, there are none.

Turning it on if you have lots of storage does nothing.

Define very low.

How many photos are in your library?

50GB may be your optimized library.