svg font is smaller than font in png?

Hi Dear all,

Quite new to SVG.

I draw a chart initially in png and now I am thinking of translate the
drawing routine to svg.

In the svg file I have
<text style="font-size: 8px; font-family: Microsoft Sans Serif;"
stroke="none" fill="#000000" x="371.608px" y="243.362px"><tspan>Aug/
09</tspan></text>

In my png image, I draw the text on with same font (8px Microsoft Sans
Serif). However, in my png, the texts look much bigger than the texts
displayed on my firefox browser. Seems if I set font-size to 11px in
svg file, I can get a relativily matched size.

Can someone help some help?

Many Thanks

Jerry