Size is a sum of:

1. bytes transferred for the request header
2. bytes transferred for the response header
3. bytes transferred for the response body

From the docs:

Inspecting the properties of an individual resource, such as its HTTP headers [note the plural, emphasis mine], content, size, and so on.

If you compare the transferSize of a performanceEntry (e.g. for the main document at https://stackoverflow.com/) with the Size in Chrome DevTools’ Network tab, you’ll see that the two don’t match up. That is because the performanceEntry doesn’t contain the size of the request header.

Screenshot of the Network tab and a logged performance entry