Hi,
There are many differnt font sizes in the default style sheet, for different elements. That's normal and reasonable. But you still shouldn't be seeing anything that might be described as unusual or out of place sizes, so a description of where you're seeing this would help. A link to your site would be even better.
As for the font-size: 62.5%; bit in the stylesheet, that's a debatable fix to a problem from the past (mostly.) (But as I implied above, that's just my opinion. I usually reset this to 80%, by the way.) You'll notice that above that in the stylesheet you see this...
font-size: 100%; /* to fix IE font bugs */
Having set the font size to 100% (of what, you might well ask) the devs chose to reset it to 62.5% of that. That will create what some people call a reasonable size for all body text and anything that is not otherwise re-sized. From that point on sizes of certain elements can be changed using EMs or % to adjust up or down fluidly, according to the user's preferences as much as anything else.
But if you insert pixel sizes later in the stylesheet, you are, for some users and some browsers, locking in a size which may be difficult for them to view.
But much of this is either old news which browsers have worked around anyway in recent versions or is irrelevant to your issue, which still needs more clarification. Where are you seeing sizes you don't like?
(You might want to take a look at some of the posts below this, down in the Similar Threads area.)
Rob



