Image Dimensions vs File Size: What Actually Changes KB and MB?
A clear explanation of pixels, compression and why two images with the same dimensions can have different file sizes.
Dimensions describe pixels, not bytes
An image that is 1920×1080 has a fixed pixel grid, but that does not tell you how many kilobytes the file will use. Two images with identical dimensions can have very different file sizes because their content and compression differ.
Complex images are harder to compress
A smooth background can be encoded efficiently. Fine texture, noise, hair, foliage and text contain more changing detail, which can require more data at a similar quality level.
Resizing and compressing solve different problems
Resizing changes the number of pixels. Compression changes how efficiently those pixels are encoded. If a source image is much larger than needed, resizing is usually the strongest first step. If the dimensions must remain unchanged, quality or format changes may be the main options.
Upload limits are byte limits
When a form says 100 KB or 1 MB, it normally refers to file size rather than pixel dimensions. A target-size compressor therefore has to inspect the generated byte size, not merely set width and height.