I am doing a page speed optimization for my website and studying how other websites do it. I noticed that some websites such as as Facebook or Ringgitplus show 0 bytes for some of their resources in Chrome's developer tools, Network tab, while the real content size is several kilobytes.
I read some articles that says that size is the amount being fetched and content is the actual size of the response. So when size is 0 bytes, it means it served from cache. But the same thing happened when I open the page using Incognito or clearing all my cache.
How is this possible and how can I achieve the same thing for my websites?