Measure full page size in Chrome DevTools
In the Network tab, make sure 'Disable cache' is checked and then when you load a page, look at the bottom panel for the total transfer size. See screenshot below:
The individual sizes are rounded, but the total at the bottom uses the byte size, which you can see in the HAR. The total then is rounded for display. If you find a slight mismatch between summing the individuals and the total shown by DevTools, that is the reason.