Reduce PNG file size on Mac

You exported a PNG and it’s 4 MB. PNGs are supposed to be efficient for graphics, but they’re not — they’re efficient for exactness. Same thing in some cases, very different things in others. Here’s how to fix the file.

Why this happens

PNG stores every pixel exactly as the encoder saw it, in 24-bit truecolour with an 8-bit alpha channel. A flat UI screenshot or design export might only use 50 distinct colours, but PNG happily allocates 32 bits per pixel to represent each one. The result: files that are 5-10x larger than they need to be.

The fix is palette quantization — re-mapping the image to use only the colours it actually needs (up to 256), stored as a colour table. This is technically lossy (the original colours might be approximated) but visually invisible on most graphics.

The native way (modest)

sips will re-encode a PNG with tighter compression:

sips -s format png —setProperty formatOptions 100 input.png —out output.png

Expect 5-15% savings. macOS doesn’t ship a quantizer.

For real savings, install pngquant:

brew install pngquant && pngquant —quality=70-90 —strip input.png

This is the same engine TinyPNG and ImageOptim use. Typical results: 60-85% smaller.

For genuinely lossless extra savings, chain on optipng:

brew install optipng && optipng -o7 input.png

Layered on top of pngquant, that’s another 5-15% from byte-perfect re-compression.

The faster way

Download Melt wraps the pngquant + zlib pipeline in a drag-and-drop interface.

1. Download Melt and open it.
2. Drag the PNG (or folder of PNGs) in.
3. Keep format on PNG and pick a quality — 80 is the standard sweet spot.
4. Click Compress. Output saves alongside originals.

The “never makes a file bigger” safety check applies per-file in batch mode, so already-optimised PNGs pass through unchanged.

How much smaller will it really get?

• UI screenshots and design exports: 70-85% smaller.
• App icons and logos with limited colour: 80-90% smaller.
• Detailed illustrations: 60-75% smaller.
• Photographs saved as PNG: 30-50% smaller (and you’d save more by converting to JPEG).
• Already-optimised PNGs: essentially unchanged.

A 4 MB PNG typically lands at 600-900 KB. A folder of fifty design exports goes from 200 MB to 30 MB. Without changing how anything looks.

When PNG isn’t the right format

The real reason a PNG is “too big” is sometimes that it should be a JPEG. Photos save as PNG sometimes because of a default that nobody changed, but PNG is the wrong format for photographic content — JPEG at quality 80 will be smaller and look the same. If your PNG is a photo, convert it. If it’s a graphic, quantize it. Different problems, different fixes.