What compression does it use?

Gzip — the same format servers and command lines use. Drop a .gz to decompress instead.

Why didn't my file shrink?

Already-compressed formats (JPG, MP4, ZIP) barely shrink again — that's how compression works, not a bug. Text, CSV and logs shrink dramatically.

No — everything happens on your device.

Compress any file with gzip (or decompress a .gz) and see the exact size savings — processed on your device, never uploaded.