I'm not sure what you mean by Windows XP themes (I do, but it doesn't really apply here... sorta). But you are probably using a shell program, to stylize your interface.
Heh.... Dangerous Dave.

with some win xp visual styles, the start button is changed. as far as the icon size, i am unsure how it is done, but it could possibly be done using icon packager or some form of icon altering program. if you want to alter the startbar yourself, check out http://wint.virtualplastic.net/ you might have to search around a little, but that site is very useful.

here's a screen of a visual style im using.

http://jingersoll99.freeyellow.com/screen.JPG

it's the Teknision visual style with ChaNinja icons. check out deviantart.com and custmize.org for stuff for customizing windows. hope that was some help.