Google真的建议手机上图标大小40dp吗？（内详）

Question

赶时间，先把我在SO的问题原样复制过来，回头再改成中文。

I carefully read the definition of dp in this page:https://www.google.com/design/spec/layout/units-measurements.html and draw a conclusion that 1dp should be representing a physical size of 1/160inch, no matter in what device. And this question http://stackoverflow.com/questions/20717887/is-dp-based-on-the-physical-size-of-the-screen-dp-1-160-of-an-inch endorses my conslusion.
Then I come across this image
on this page. According to the page, the gray circular should be 40dp, which is 0.25 inch. In this fiddle you can see actual 0.25inch comparing to the 40dp in this image.
I ask myself, is this image being enlarged so the 40dp on it looks much bigger than 0.25inch? I compare the image to my nexus 6, on that page they are about the same physical size. And the elements on this picture look normal as if they are in a real device. Which means, if I implement this 40dp/0.25inch circular icons on my cellphone, they are definitely going to look horribly small. I understand that sometimes 1dp might be a little different from 1/160 inch, but it won't shouldn't be a huge difference lke this. This is where I got confused and had to come here asking for help.
Did google really suggest that the gray circular be 40dp and 0.25inch?