HEIC · Image formats
Why your converted image got bigger
Because converting changes how an image is stored, not how much detail it contains, and some formats store the same picture far more efficiently than others. Going from HEIC to JPG typically produces a file 1.5 to 2.5 times larger, because HEVC compression is simply better than JPEG. Converting anything photographic to PNG produces something much larger again, because PNG is lossless and stores every pixel exactly. The counterintuitive case is a flat graphic or screenshot converted from PNG to JPG, which can also grow, because JPEG handles sharp edges and flat colour badly. If your goal is a smaller file, the lever is pixel dimensions, not format.
Format is a storage decision, not a quality one
It helps to separate two things that get conflated. The image is a certain number of pixels of a certain complexity. The format is a scheme for writing those pixels to disk compactly. Converting swaps the scheme while leaving the picture the same.
So the size change is entirely a question of which scheme is better at describing that particular image. A newer format is usually better, which is why converting forward tends to shrink files and converting backward tends to grow them. Nothing is being added; the description is just less efficient.
The usual culprits
HEIC to JPG is the most common. HEIC uses HEVC compression, which is roughly twice as efficient as JPEG at matching quality, so undoing it costs you that factor. Expect 1.5 to 2.5 times the original size. This surprises people converting an iPhone photo to satisfy an upload limit, and it is precisely the wrong move for that problem.
WebP to JPG is milder but the same story — WebP is 25 to 35 per cent more efficient, so expect the JPEG to be correspondingly larger.
Anything to PNG is the dramatic one. PNG is lossless: it stores every pixel exactly rather than discarding detail the eye will not miss. On a photograph, where almost every pixel differs slightly from its neighbours, that is enormously expensive. A 2 MB JPEG photograph can become a 15 MB PNG without gaining a single visible detail.
The counterintuitive case: PNG to JPG can grow
Everyone expects JPG to be the small one, so this catches people out. Convert a screenshot, a logo, a chart or a diagram from PNG to JPG and the file can come out larger than it went in.
The reason is that the two formats are good at opposite things. PNG compresses by finding runs and patterns of identical pixels, so a screenshot with large areas of flat white and crisp black text compresses spectacularly well. JPEG works by discarding high-frequency detail, and sharp edges are pure high-frequency detail — so it has to spend a lot of data trying to reproduce them, and does it badly, surrounding your text with faint ringing artefacts.
So for flat graphics, PNG is both smaller and better looking. For photographs, JPEG is both smaller and perfectly adequate. Choosing by reputation rather than by content is what produces the surprise.
Artefacts get preserved faithfully
There is a subtler version of the problem worth knowing about. When you convert a lossy image to a lossless format — a WebP or JPEG to PNG — the compression damage already present in the original becomes part of what gets stored exactly.
PNG cannot tell the difference between real detail and JPEG ringing around a letter. It sees pixels that differ from their neighbours and dutifully records every one at full fidelity. The result is a large file that faithfully preserves flaws, and it does not look any better than the original because no information was recovered.
Converting to a lossless format does not undo lossy compression. It only stops any further loss from that point on.
What actually makes a file smaller
Pixel dimensions, by a wide margin. File size scales roughly with the square of the dimensions, so halving both width and height cuts the size to about a quarter. A 4000 pixel photograph resized to 1000 pixels is around a sixteenth of the size, and for a document upload or a web page it is very likely still more detail than needed.
Quality settings help within a format. JPEG at quality 0.92, which is what this site uses, is visually indistinguishable from the original on a photograph. Dropping to 0.7 saves substantial space and is fine for most web use. Below about 0.5 the artefacts become obvious.
And choosing a more efficient format helps if the destination accepts it — WebP over JPEG, AVIF over WebP. That only works where you control what reads the file, which is why it is the answer for a website and not for an upload form.
What does not help: converting HEIC to JPG to get under a size limit, or converting to PNG for any reason involving size. Both reliably make things worse.
Frequently asked questions
Why is my JPG bigger than the HEIC it came from?
Because HEVC compression is roughly twice as efficient as JPEG. Converting gives up that efficiency in exchange for a format that opens everywhere. A factor of 1.5 to 2.5 is normal and is not a fault in the conversion.
Why is my PNG so much larger than the JPG?
PNG is lossless and stores every pixel exactly, while JPEG discards detail the eye is unlikely to miss. On photographs that difference is large — a five to ten times increase is common and no visible detail is gained.
Why did converting my screenshot to JPG make it bigger?
Because JPEG is bad at sharp edges and flat colour, which is what a screenshot is made of. PNG compresses those patterns extremely well. For screenshots, diagrams and logos, PNG is usually both smaller and sharper.
How do I actually reduce a file to fit an upload limit?
Reduce the pixel dimensions. Size falls with roughly the square of the scale, so halving the width and height gets you to about a quarter. Changing format alone rarely helps and converting to PNG or from HEIC to JPG makes it worse.
Does converting to PNG improve the quality of a JPEG?
No. It preserves exactly what is there, artefacts included, at a much larger size. Lossless does not mean restored — nothing that was discarded comes back.
Which format gives the smallest file?
AVIF, then WebP, then JPEG, for photographs. For flat graphics and screenshots, PNG usually beats JPEG on both size and appearance. The best choice depends on the content, not on the format alone.