I believe that the size is what was sent over the network (including the http headers), and the content represents the total size of the content.
For example, if I hit a page that 304 responds to without being resized, these are just a few bites, while the content is much larger, because only the response header 304 was sent over the network.
In addition, the size may be smaller if the content was archived before it was sent over the network.
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