How is google getting 2.7MB? Are they also fetching the third party URLs in discussion threads? Or maybe they mean median session?
I pointed the tool at a Tumblr blog, and I see: "PSI estimates this page requires 6 additional round trips to load render blocking resources and 1.3 MB to fully render. The median page requires 4 round trips and 2.7 MB. Fewer round trips and bytes results in faster pages."
I think 2.7MB is the size of the average web page on the Web on mobile (3.4M on desktop). Which is quite scary. (Edit: though this may refer to pages that are being developed and tested in Page Speed, see the other comment).
I wrote a whole (private, small) website with pictures (photos and images) and styles that includes a maze using JavaScript in 0.6 MB total. I didn't spend too much time in optimizing this.
I cheated a bit: links in this website point to anchors in the same HTML file. This does mean that without this trick, one page would be even lighter.
I would find it hard to write a 3MB webpage without doing it on purpose. Something is wrong with Web development. Stop wasting resources!