I just took two photos on my smartphone. One of a table surface with nothing on it and another of a cluttered table top full of detail. The clear photo was 3.5 MB and the cluttered one was 5 MB. How is this possible? The resolution is the same so they have the same number of pixels. Why would one create a larger image file than the other?
Answer
This is not only possible, but extremely likely, when you're using a compressed image format such as JPEG. Data compression methods in general become more efficient as the data to be compressed decreases in entropy (try creating zip files of a large page of actual text vs. the same sized page of a single repeated character).
The more features or fine textured detail that an image contains, the less the compression can "cheat" by simplifying the data that it stores to represent the image when it's uncompressed. On the other hand, cameras that store images as uncompressed RAW files tend to produce similar-sized files per image, regardless of image content.
A lossy format like JPEG, because it actually discards some inessential data in a very clever way while compressing, can often achieve ratios up to 10:1 while still giving a decent representation of a typical photograph, while a lossless compression format that retains all data might only achieve 2:1 compression, or less.
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