If you've noticed on my blog I've heavily increased my font size for body copy & titles. It's not that making it bigger makes it more important, that is absolutely not the case. However making it larger than my secondary information does emphasize it's importance more along with increasing the readability comfort.
I only wanted between 12-15 words per line so I just kept playing with the font sizes until I got around there. Since the web offers pretty limited type controls I tend not to sweat it so much.
Though it is interesting that I've never thought to try that before. I feel as though since 12px font sizes were used on every website that was the "standard" but lately I've been questioning things like that. 12px isn't bad by any means but it's not always best. I usually use around 14px via the ems measurement. Thus my secondary information gets to be 12px which is still comfortable to read.
However on my blog a few designers I know mentioned that my initial blog design was sort of hard on the eyes with so many words and the negative type (white text on black backgrounds). My first attempt to fix it was to reduce the size of the content area for blog posts, this worked but then it created an unpleasantly large gap between the sidebar and body content. So that's when I realized I could just increase the font size and it will make it infinitely easier to read on any platform plus maximizing the readability by having a decent amount of words per line.
Anyway I suppose this article is a bit dull. So I'm saying the takeaway from this is that just because you see something commonly used in a design in any medium, doesn't mean you should just accept it. Question "Why?" is it common. For instance when thinking about the font size I realized the reason why most sites use 12px is that it's the smallest font size that's still pleasant to read which allows them to cram more content on screen. I am not concerned with that therefore there's no reason for me to follow that norm.