Project size vs File size

Why does my file size after export show a folder of 25MB in size but Nomad Sculpt viewport shows 779MB?

Initially, I suspected that the large PNG texture files were the cause, but after converting all files to JPG, saving, and reopening the project, the size is still 779MB. I understand this might be a common problem with similar applications, but I’ve always been curious about the underlying reason for this.

***This model is one of my environment sets for a Lycan based horror film still in production. It’s noisy because I keep my samples at 50. And, I’m not a fan of the new denoiser feature. I like using emissive objects as my main source of light and only a hint of environmental light when working on gritty projects like this. iPad Air M2 (8GB) ***