When pictures are uploaded directly into LR CC duplicates of the files are made (stored in pictures folder for mac or app data folder for windows), even when the original is already stored locally. This seems completely unnecessary and takes up twice as much space.
The logic is that LRCC must be able to guarantee it's still able to access the file when it gets chance to upload it. You can change the location of the originals in the Preferences dialog though, so you could redirect it to another drive.
I have to agree that the logic is not very strong. Because you can change the location of the local storage in Lr CC, you could even choose the same folder as where the images are currently located. And still Lr CC would duplicate them (if I understand it correctly)! That doesn't seem to make a lot of sense, and for people with a large Lightroom Classic catalog that they want to migrate, it might even mean they have to buy a new external drive, just so Lightroom CC has enough space to duplicate all the images. Lightroom CC should have an 'ADD' option that uses the images in their existing location, just like Lightroom Classic has.