Mac storage basics · Apps and macOS itself
Why does an app take up more space than its download size?
Short answer: Downloads are compressed, and after installing, an app's caches, downloads and data grow in your Library — so its real footprint is often far larger than the app itself.
Downloads are compressed, so an installed app is usually bigger than the file you downloaded. After that, the app itself stays roughly the same size, but the data it creates grows: caches, downloaded content, offline files, logs and documents, mostly stored in your Library rather than inside the app.
That's why an app listed at a modest size in Applications can be responsible for much more. Storage Studio's What's Using Space attributes that data to the app it belongs to — see What's Using Space — and the Application Support folder explains where it usually lives.