Nope. Not in terms of the way font size is specified it isn't. As mentioned above, 12px means that the distance from the bottom of letters such as p, g, q and y to to the tops of letters such as t, f, A etc. is 12px. If you take a screen grab of some 12px Arial beside some 12px Verdana, you will find that each measures 12px.
What is definitely much larger in Arial than in Verdana is the x-height [en.wikipedia.org] of the letters. In Firefox at least--the only place I looked--the default line-height for Arial seems also to be 15px versus Verdana's 14px. This could also make a noticeable difference over a hundred lines or so of text.