Notes
Why gradients and grain double your PNG file size
Two files, both 4500 × 5400, both transparent PNG. One is 2 MB. The other is 50 MB. Nothing about the dimensions explains the gap — the content does.
PNG predicts, then stores the error
PNG compression works row by row. For each pixel it guesses a value from the neighbours already encoded — the one to the left, the one above, or a blend — and then stores only the difference between the guess and reality. A flat area of colour is guessed perfectly every time, so the stored difference is a long run of zeros, and runs of zeros collapse to almost nothing. That is the whole trick.
Noise is unguessable by definition
Now consider film grain, a paper texture, or a subtle distressed overlay. Each pixel differs from its neighbours by a small random amount. The prediction is wrong every single time, by a different amount every time, so the stored differences are themselves random — and random data does not compress. You end up storing something close to the raw pixel values. That is the 50 MB file.
Same canvas, different content
| Flat shapes, hard edges | compresses extremely well |
|---|---|
| Smooth gradient | compresses well — the prediction is nearly right |
| Gradient plus grain overlay | compresses badly — the grain defeats every prediction |
| Photographic texture | compresses badly |
What to do about it
If the design needs texture, keep it and reduce the colour count instead — quantising to a smaller palette cuts the file dramatically and is usually invisible on flat illustration. If the texture is decorative rather than essential, removing it is often the single largest saving available, and it costs nothing in print quality. Either way, check the result rather than guessing: the same visual change can save 30 MB on one design and 3 MB on another.
Frequently asked questions
Would saving as JPEG fix the size problem?
Not for apparel. JPEG has no alpha channel, so the transparent background becomes solid white or black and prints as a visible box on a dark garment. The size ceiling and the transparency requirement have to be satisfied together.
Does reducing the canvas size help?
It would, but on Merch by Amazon the dimensions are mandatory — exactly 4500 × 5400. You have to reduce bytes with the pixel count locked, which is why colour reduction is the lever that remains.