Three Different Levers, Often Confused
People say "compress" when they mean one of three quite different operations, and choosing the wrong one is why results disappoint.
Resizing changes the pixel dimensions. Going from 4000 × 3000 to 2000 × 1500 removes three quarters of the pixels before any encoding happens. This is the largest single saving available and usually the least visible, because almost no destination displays images at full sensor resolution.
Quality reduction changes how precisely the encoder stores each block. JPEG divides the image into 8 × 8 blocks, transforms each into frequency components, and rounds the high-frequency components more coarsely. Lower quality means coarser rounding.
Format change swaps the encoder entirely. WebP at the same perceived quality is typically twenty to thirty per cent smaller than JPEG because it uses better prediction and entropy coding.
Target Sizes That Actually Make Sense
| Destination | Long edge | Quality | Typical result |
|---|
| WhatsApp or email attachment | 1600 px | 0.80 | 200–400 KB |
| Blog or article image | 1200 px | 0.80 | 120–250 KB |
| Full-width banner | 1920 px | 0.75 | 250–500 KB |
| Government form, 1 MB limit | 1200 px | 0.75 | Comfortably under |
| Passport-style photo upload | 600 px | 0.85 | 40–90 KB |
| A4 print | 2480 px | 0.92 | 1.5–3 MB |
A Worked Example
A 4.2 MB phone photo at 4032 × 3024 must go under 300 KB for an online application form.
- Resize the long edge to 1400 px. Pixel count falls from 12.2 million to about 1.5 million, roughly one eighth.
- Encode at 0.82. Expect somewhere near 240 KB.
- If it lands high because the image is unusually detailed, drop to 0.75 rather than resizing again. Detail loss from quality reduction at this level is far less noticeable than losing another third of the resolution.
Note the order. Resize first, then adjust quality. Doing it the other way round means you compress data you are about to throw away.
Practical tip: always work from the original. Each JPEG save re-quantises the image, so compressing an already-compressed file stacks artefacts. If you need three different sizes, produce all three from the source rather than shrinking the shrunk version.
Choosing the Right Format
JPEG is the default for photographs and opens in everything ever written. It has no transparency and handles sharp edges poorly.
PNG is lossless and correct for screenshots, diagrams, logos and anything with transparency. Used on a photograph it will often produce a file larger than the uncompressed original JPEG, which surprises people.
WebP is the best general choice for the web today, supported by every current browser, with both lossy and lossless modes and transparency support. Some desktop software and older email clients still cannot open it, so it is a weaker choice for attachments.
Why Your File Manager Disagrees About Sizes
Two discrepancies come up repeatedly. First, storage is measured in powers of two by operating systems and powers of ten by manufacturers, so 1 MB may mean 1,048,576 or 1,000,000 bytes depending on who is counting. Second, file managers often report space occupied on disk rather than actual file size, which rounds every file up to a whole number of allocation units. A folder of many tiny images can occupy noticeably more disk than the sum of the file sizes.
Mistakes That Cost You
Aggressive quality with unchanged dimensions
A 4000-pixel image at quality 0.5 looks worse and is often larger than a 1600-pixel image at quality 0.85. Resolution is the more efficient lever.
Compressing screenshots as JPEG
Screenshots are dominated by sharp, high-contrast edges, exactly what block-transform compression handles worst. Text acquires a grey halo. Use PNG or lossless WebP.
Crushing document scans
Small print is precisely the fine detail that quantisation destroys first. Keep scans at higher resolution and higher quality than holiday photographs, and read the smallest text on the result before sending it.
Assuming metadata is removed
Compression does not automatically strip EXIF data. A resized photo may still carry the GPS coordinates of where it was taken. If you are publishing publicly, verify that.
What Compression Cannot Do
It cannot sharpen a soft image, recover blown highlights, or make a small image suitable for large print. It cannot meaningfully shrink a file that is already heavily compressed — attempting it just layers new artefacts on old ones. And it is one-directional: no process restores discarded detail, whatever an application claims.
Runs Entirely In Your Browser
Decoding, resizing and re-encoding all take place on a canvas inside the page. Your images are never transmitted. Open developer tools, watch the Network panel while compressing, and nothing leaves. Or disconnect from the internet once the page has loaded and compress anyway.
That is the point for the files people most often need to shrink: identity documents, address proofs, medical scans, signed contracts and photographs of children for school portals. Those should not be passing through infrastructure you cannot inspect.
More Questions Answered
Should I resize or reduce quality first?
Resize first, always. Cutting the pixel dimensions to what the destination actually displays removes far more data with far less visible cost than aggressive quality reduction on a full-size image.
What quality setting is safe?
Around 0.80 to 0.85 for general use, and 0.90 or higher when the image will be printed or contains small text. Below about 0.70 you start seeing blocky artefacts around high-contrast edges.
Can I undo compression later?
No. Lossy compression permanently discards information. Keep your originals somewhere before processing a batch.
Why is my PNG bigger than the JPEG it came from?
PNG is lossless and is designed for flat colour and sharp edges. On a photograph, where every pixel differs slightly from its neighbours, it has almost nothing to exploit and files grow accordingly.
Is WebP worth using?
For websites, yes — it is typically twenty to thirty per cent smaller than JPEG at the same visible quality and every current browser supports it. For email attachments JPEG remains safer, since some older software cannot open WebP.
Does compressing remove location data from my photo?
Not by default. Metadata is only stripped if the tool explicitly does so, so check a sample before publishing photos you would rather not geotag.
Why does my file manager report a different size?
Either because it is showing space occupied on disk, which rounds each file up to a whole allocation unit, or because it counts a megabyte as 1,048,576 bytes rather than 1,000,000.
Are my images uploaded anywhere?
No. All processing happens inside the browser tab. You can verify it in the Network panel of developer tools, or by disconnecting from the internet before compressing.