Hi,
With percent based font sizes, it has to work against a reference fixed size, in this case the base size it trying to be set here:
.tagcount { font-size: x-small;}
but that elector may not be specific enough if you have other font sizes set on anchor elements elsewhere in the css
You may need to try stuff like this:
.tagcount a { font-size: x-small;}
.tag-cloud .weight1 a { font-size: 90%; }
.tag-cloud .weight2 a { font-size: 110%; }
.tag-cloud .weight3 a { font-size: 120%; }
.tag-cloud .weight4 a { font-size: 130%; }
.tag-cloud .weight5 a { font-size: 140%; }
.tag-cloud .weight6 a { font-size: 150%; }
.tag-cloud .weight7 a { font-size: 160%; }
.tag-cloud .weight8 a { font-size: 180%; }
.tag-cloud .weight9 a { font-size: 200%; }
.tag-cloud .weight10 a { font-size: 210%; }
Maybe that will solve it.
Hope it helps,
Joe