Right, ckv, often it's the many colour nuances created when resizing due to antialiasing which will not compress that well anymore.
Some more aspects when talking about file size and image compression:
- JPG is a lossy format. Most JPGs available have been compressed with loss, i.e. the saved image is a bit different from the original and will have artefacts and be smoothened.
- Changing an existing JPG image and then saving it might result in larger file size since the original image has been well compressed with loss (artefacts, smoothening)
- Resizing creates new colours depending on the resize algorithm.
- Compression algorithms have different results on the same image data, changing the compression algorithm might result in a big change in file size.
- There's two ways to save the colour information of an image:
1. Colour palette (image data are only indexes in the colour palette)
2. Colour value per pixel (true colour or RGB image).
If you have few colours, using a palette can be much smaller than using true colour. Using a palette depends on the image format, e.g. GIF uses colour palettes.
I'm sure there's more aspects. This area is fairly complex, but always explicable.
A rule of a thumb is:
Significantly smaller images (e.g. 50% (or less) of original width and height) are very likely to be smaller in file size.
Nowadays, most people have monitors with a resolution of 1024x768 pixels. So when sending an image via e-mail and viewing it on the screen, it shouldn't be larger than that, sure enough the smaller the better. Personally I found that 800x600 is often a good value.
Some references to existing explanations and posts that also provide interesting info regarding the "size phenomenon":
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Smaller image has same file size
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Much larger file when converting JPG to BMP
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Resizing image results in larger file
Drahken
Hello !
VG