Images
Resizing vs compressing an image: what’s the difference?
Resizing changes the pixel dimensions, compressing changes how the pixels are stored. We tried both on the same photos to show when to use each, and when to use both.
Two different knobs
- Resizing changes the number of pixels: a 4000 × 3000 photo becomes 2000 × 1500. The picture gets physically smaller on screen and in print.
- Compressing keeps the same pixels but stores them more efficiently, usually by throwing away detail the eye barely notices. The picture stays the same size; the file gets lighter.
Both make the file smaller, which is why they are often confused. But a form that says “maximum 600 × 600 pixels” needs resizing, and one that says “maximum 2 MB” usually needs compressing.
What each did to the same photos
We used 9 public sample photos from the scikit-image project, between 451 and 1,411 pixels wide.
- Resizing the PNG versions (6,072 KB in total) to 50 % width and height with Bulk Image Resizer left a quarter of the pixels, and the files came to 2,371 KB.
- Compressing the JPG versions (1,133 KB) at quality 75 with Compress Image kept every pixel and brought them to 550 KB, 51 % smaller.
- Simply saving the PNG versions as JPG at quality 75, without resizing, brought 6,072 KB down to 550 KB.
The lesson: for photos stored as PNG, changing the format does more than halving the size. For photos already in JPG, compression still roughly halves them.
When to resize
- A form or platform asks for exact pixel dimensions, such as a profile picture or an ID photo.
- The image will only ever be shown smaller than it is. A 4000-pixel photo in a 800-pixel-wide blog column wastes bandwidth on every visit.
Avoid enlarging. Making a small image bigger adds pixels but no detail, so it looks soft. Bulk Image Resizer doesn’t enlarge smaller images unless you untick that option, and when you enter both width and height it keeps the proportions so nothing gets stretched.
When to compress
- An upload or email limit is in megabytes, not pixels.
- The image is already the right size but the file is heavy.
- You are saving many photos for a website, where every kilobyte counts.
For JPG, quality 75 to 85 suits most photos. Below about 60, blocky patches start to show in skies and skin. PNG has no quality setting in browsers, so to compress a PNG photo, convert it to JPG or WebP with JPG PNG WebP Converter or choose a different output format in the compressor.
Often, do both, in this order
Phone photos for the web or for a document usually need both: first resize to the size it will actually be shown at, then compress. Compressing first and resizing afterwards means the image is re-encoded twice, which costs quality for no benefit. Compress Image can do both in one go: set Max size to the longest side you need, and pick a quality.
Everything happens on your device, so the original stays untouched and private.