PNG compression is a different sport
JPEG compression throws away picture information. PNG compression, at its best, is lossless: it finds a cheaper way to store the same pixels (filters, zlib, indexed palettes). A logo with 12 colors does not need 24-bit PNG; an 8-bit palette PNG can be a tenth the size and look identical. A screenshot of a terminal is mostly flat color and compresses well. A PNG of a photograph will not shrink like a JPEG would, because lossless cannot discard grain. If your “PNG” is a camera photo, converting to JPEG or WebP is the real compressor. This tool is for graphics that must remain PNG.
ToolPin may use palette reduction tied to a quality-like control. That can become visually lossless for icons and slightly lossy for photos. Inspect the result. If you see banding in a sky, you needed a format change, not a tighter PNG.
Transparency and chunks
A good PNG compressor keeps alpha. That is the reason you did not flatten to JPEG. Soft shadows should remain. Some optimizers strip ancillary chunks: text comments, time stamps, maybe ICC. That is usually fine for the web and good for privacy. If a print workflow depended on an embedded profile, keep the original PNG.
APNG animation is not the focus of a still compressor. You may get a still or a flattened result. Keep a copy of animated PNG elsewhere.
When size barely moves
Busy photographic PNGs are already near the lossless ceiling. Further PNG compression saves a few percent. The honest options are: resize, convert to WebP/AVIF/JPEG, or accept the weight. Also check dimensions—a 3000-pixel screenshot of a 900-pixel-wide article is the real bug.
Tiny 16×16 icons may grow if re-encoded poorly. Compare bytes every time.
Privacy and limits
Everything is in-browser. Unreleased UI and product mockups stay on the device. 250 MB cap. No watermark, no account. Metadata in tEXt/iTXt chunks is often dropped on re-encode, which is helpful before you publish a screenshot that still had your username in a comment field.
Device memory matters for huge @3x marketing PNGs. Resize toward the largest displayed size first.
A sane PNG diet
Masters: keep 24-bit PNG or SVG in the design folder. Delivery: compressed PNG for places that require PNG (App Store screenshots, some emails, Git READMEs that cannot do WebP). Site: WebP/AVIF with PNG fallback. Photos: do not leave them as PNG. That diet, plus this compressor for the PNG you must ship, is how you stop shipping 4 MB “simple” graphics.