Number of Bytes Dramatically different in different page loads for the same website

I have noticed as I have been running tests that sometimes, the number of bytes downloaded during a page load varies significantly, even when the same website is simply loaded multiple times. For example, looking at http://www.webpagetest.org/result/180104_6Z_9a7a333b9db169a0d72da46bd2b89683/ the number of bytes downloaded varies by up to 700MB even from one run to the next. What could be causing this variation?