Why is 13px Arial font the web standard?

If you use any of Google's applications, from search to Gmail you will find the base font used for most things is 13px Arial.

font-family: arial, sans-serif;
font-size: 13px;                       

I always thought 16 pixel fonts were easier for the "masses" to read. However, Google seems to disagree - and they are not alone. Twitter's Bootstrap project and many others also use 13px fonts. Even the text of this very site is only 1px bigger.

Do web browsers have it wrong, is the proven standard font size really 13px and not 16px? What about mobile devices - which works best on them?

Note: Serif fonts (Georgia) need larger sizes than Sans-Serif (Arial). However, most everyone agrees that Sans-Serif are easier on the eyes in digital format while Serif are easier to read in printed media.

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Edit: Is there research or usability tests that back 13px? What evidence supports this trend? Where can I learn more about the pros/cons or applicable states for this size?