Why are my processed images larger than the originals? Do they have to be?
No, they don’t have to be. File size depends on these factors:
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Number of pixels,
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file format (compression level)
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layers
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CMYK versus RGB
Number of pixels: do you resample images as part of your processing? (SIZE settings). Do you really need to resample and are your settings correct? (to resample or not to resample). Since camera resolutions have become very high, correct resampling typically leads to smaller images rather than larger, but of course this is only a rule of thumb. Many exceptions to this rule may exist.
File format (compression level): Most people know that a jpg is smaller than a tiff or psd due to compression. Our default jpg quality level is 9, on a scale from 1-10, where 10 represents the highest quality and the least compression. Level 10 has very little compression, and therefore a relatively large file size. The difference with level 9 is substantial, and if file size is an issue, 9 is typically a safe choice to make.
Layers: Some file formats support multiple layers. (PSD and Tiff). Each layer is as big as the image itself. The original file can have multiple layers. You can output that as a single layer file or you can keep the original layers and add a processed layer. Optionally you can also add a compatibility layer. All these layers can of course lead to very large file sizes. Especially since formats that support layers (tiff or psd) are uncompressed or lossless compressed.
CMYK versus RGB: A CMYK image is larger than an RGB image (all else equal). For each pixel and each color channel there is a byte of information. Therefore an RGB image has 75% of the information that the CMYK version has.