Why a PNG Converted From JPG Is Often Much Larger
Compare lossy JPEG and lossless PNG behavior to understand why a simple format change can increase file size dramatically. A JPEG has already used a lossy model optimized for photographic content. Encoding the decoded pixels losslessly as PNG can require more bytes, especially when the image contains photographic noise and gradients.
Find the cause before changing settings
A JPEG has already used a lossy model optimized for photographic content. Encoding the decoded pixels losslessly as PNG can require more bytes, especially when the image contains photographic noise and gradients.
Start by reproducing the problem with the original file intact. Record what is wrong in concrete terms: page order, clipping, transparency, blur, file size, or a destination rejection. Then inspect lossy JPEG compression before changing anything.
A technical detail worth checking
If JPEG compression blurred a one-pixel line into several neighboring pixels, PNG will preserve those blurred pixels exactly. It cannot infer the original line that existed before JPEG quantization.
What the file is telling you
lossy JPEG compression: JPEG commonly uses lossy transform coding for photographic images. Once detail has been discarded by a previous encode, changing the container format cannot recreate that source detail.
lossless PNG compression: PNG is designed for lossless raster storage and can preserve exact decoded pixel values, but lossless storage can be larger than JPEG for noisy or photographic content.
pixel dimensions: Pay attention to pixel dimensions. It changes how the source is interpreted or how the output is laid out, so verify it in the final file rather than assuming the preview and the exported result are equivalent.
target file size: Pay attention to target file size. It changes how the source is interpreted or how the output is laid out, so verify it in the final file rather than assuming the preview and the exported result are equivalent.
A controlled conversion workflow
- Make a copy of the source and identify the destination rule.
- Inspect lossy JPEG compression and lossless PNG compression before conversion.
- Run JPG to PNG once with a deliberate target instead of stacking repeated conversions.
- Open the result independently and check pixel dimensions and target file size.
- Keep the original until the derivative has passed the acceptance test.
Acceptance test: prove the output works
- Check lossy JPEG compression against the source.
- Check lossless PNG compression at the size the recipient will actually use.
- Inspect target file size and any page/image edges where errors tend to become visible.
- Confirm the output meets the real destination rule, not merely that the file opens.
Shortcuts that usually backfire
- Treating lossy JPEG compression as cosmetic when it actually changes output geometry or encoding.
- Optimizing lossless PNG compression without checking what happens to readability or structure.
- Assuming a smaller or visually similar file is automatically the correct deliverable.
- Converting the converted result again instead of returning to the original source.
Use JPG to PNG after the source has been checked. Convert once, then inspect the actual derivative before it enters a portal, email, client package or archive.
Frequently asked questions
What is the first thing to check for this JPG to PNG problem?
Start with lossy JPEG compression. Then compare it with lossless PNG compression. Those two checks usually tell you whether the issue comes from the source, page/image geometry, or the conversion target.
Can changing the file format alone fix the problem described here?
Not necessarily. Format conversion changes representation; it does not automatically repair source defects. If lossy JPEG compression is wrong before conversion, validate it explicitly after conversion.
What should I compare in the output?
Compare the output against the original job, not only against the source preview. Pay special attention to lossless PNG compression and target file size, plus page order, readability, dimensions and required file size.
Why keep the original file after a successful conversion?
Because the original is the reference for corrections and re-export. If a later requirement changes, working from the source is safer than repeatedly converting an already converted derivative.
Related JPG to PNG guides
Technical references
JPEG Committee — JPEG 1 overview ↗
W3C — PNG Specification, Third Edition ↗
Last reviewed September 14, 2026.