What is taking up space on my drive?

I have a 1TB hard drive in my pc that only stores recordings from NVIDIA game capture. It reached full capacity when I recorded about 1500 files. So, to free up space, I deleted a couple hundred. I kept clipping new videos, and noticed that it filled up faster than it should have. Deleted more files - same thing - filled up way too quick. I checked how many files there are, and it's about 900.

It used to be full with 1500 files, now its full at only 900. How? The average size of one of those clips is like 100MB, with only a few reaching the size of 1GB.

What is taking up the rest of that space? I've tried doing disk cleanup to clear the garbage can and also delete shadow copies, but it didn't free up any space. How do I fix this?