How to "configure" my IE 8 to keep my fonts sign??

I have XP Pro installed and sometimes I would like to have my IE 8's fonts sign temporarily increased by pressing "Control +" for a given web sites to make my viewing easier! However my IE 8, most often, will automatically resetting itself back to its normal fonts sign (smaller) after I have press "Control +". My Firefox's has not got such a problem because it can keep its setting after pressing "Control +" in Firefox windows. So why is it happening for my IE 8 and how to configure it to behave the SAME as my Firefox"??
Mandy thanks