Why a compressed PNG is no longer a PNG
PNG is a lossless format. It stores every pixel exactly, so there is no quality setting to lower. ToolGPT's compression therefore writes the image as JPEG or WEBP. Both are lossy formats that leave out fine detail the eye barely notices, and that is where the saving comes from. WEBP is usually smaller than JPEG at the same quality.
How much is saved depends on the image. A photograph stored as a PNG can shrink to a small part of its size. A screenshot or a flat diagram with large areas of one color may shrink very little, and sharp text can look softer as a JPEG. Each option shows the size it will not exceed, so you can judge before you tap.