Quality, dimensions, and file format
Image size is controlled by three main factors: pixel dimensions, encoding quality, and format. JPEG and WebP can reduce detail gradually. PNG is lossless, so meaningful reductions often require fewer pixels or a format conversion.
The compressor tests settings locally and keeps the best result it can produce under your target. If quality changes are not enough, aggressive modes may reduce dimensions. The result shows the final format and size before you download it.