I exported a 7109 pixel X 4742 pixel photo. When I did this, I asked Lightroom to resize to fit the image into a size of 1024 X 764 pixel. As expected, the longest size became 1024 pixels and the shorter side became 683 pixels (longest side reduced about 6.94 times and so was the shorter side keeping its original proportional size). When I multiply the 1024 by 683 to get the size of the file, I would expect about 699Kb for the file size, but the file size was only 594Kb. What happened? Does Lightroom use some type of compression or am I missing something like the file size does not directly relate to the pixel number?
You're confusing pixels and bytes. There are 24 bits per pixel for an 8 bit file, 48 for a 16 bit file. Plus overhead.
But yes, some file formats compress the data - most notably jpeg, which uses very aggressive (and irreversible) compression, to get file size down to perhaps 2-5% of original size.
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