PNG doesn't negotiate with image data. Every pixel that goes in comes back out exactly as it was, which is the entire reason the format exists, and it's also exactly why converting an efficient, modern codec like AVIF into PNG produces such a lopsided result on disk. AVIF earns its small file sizes by making calculated decisions about which visual information the human eye won't miss. PNG refuses to make any of those decisions. Ask it to store the same image and it stores all of it, discarding nothing, which naturally costs far more space than AVIF ever needed to spend getting there.

That gap is larger than it looks on paper, and larger than the equivalent AVIF to JPG conversion produces. JPG is also a lossy format, so converting to it still involves some data being thrown away on the way out, softening the size increase somewhat. PNG throws nothing away, so the full weight of everything AVIF managed to compress lands back on the file the moment it becomes a PNG. The size increase in practice runs noticeably heavier than what you'd see converting the same AVIF straight to JPG, precisely because PNG has no compression trade-off left to offer.

What that trade buys you is worth being specific about, because it isn't nothing. AVIF supports transparency, and so does PNG, so a source file with a transparent background keeps that transparency fully intact through the conversion, alpha channel and all, with none of the flattening that happens converting the same file to JPG. If the AVIF you're starting with carries higher bit depth than the standard 8 bits per channel, PNG is technically capable of preserving that too, since the format supports up to 16 bits per channel, well beyond what JPG can hold. Whether that extra depth survives in practice depends on how the conversion is processed, since most browser-based image pipelines, including the canvas rendering most converters rely on, typically operate at 8 bits regardless of what the source file was capable of storing. For the vast majority of AVIF files circulating on the web today, which were already encoded at standard 8-bit depth to begin with, that limitation doesn't cost you anything you had in the first place.

None of this makes PNG the wrong destination. It makes it a specific one. Converting to PNG is the right call when the image needs to be edited further, layered into other design work, or archived somewhere size genuinely doesn't matter and precision does. It's the wrong call if the plan is simply to get the file working somewhere AVIF isn't supported, where JPG or WebP will get there with far less bloat. The AVIF to PNG converter runs entirely in the browser either way, so testing which outcome actually fits a given file costs nothing but a few seconds.