How can a PNG get smaller without losing quality?
PNG is lossless: it stores every pixel exactly. But the same pixels can be written in many ways. PNG filters each row of the image, then compresses the result with DEFLATE, and most apps save with fast, middle-of-the-road settings. oxipng tries different filters and compression strategies and keeps whichever is smallest. It also drops chunks the image doesn't need to display, like editing-software notes.
The result decodes to precisely the same pixels. That makes it safe for screenshots, UI assets, diagrams and anything with text, where lossy compression would show.
When a PNG should become something else
Photos saved as PNG are enormous because lossless compression can't do much with camera noise. If a PNG is really a photo, converting it to JPEG, WebP or AVIF will make it many times smaller. That's a Squeezels Pro feature. For graphics with transparency headed for the web, WebP keeps the transparency at a fraction of the size.
