Fonts and amount of bbcode tags used is not counted. Only "physical" size of the output matters, what is seen on the preview. The result you get is odd.
I checked it and your current sig behaves just right in the sig editor. I can actually fit 3 times your current signature there.
Couple of thoughts:
Have your tried it on another PC?
If you zoom in/out the browser, the sig size validity check will get wrong results. Try 100% zoom.
Empty lines count, too. check that you do not have lots of empty lines after the sig.
PM me the sig that you want and I can set it up for you until we can solve this.