Scaling my thumbnails

amount = 2;
for (i=0;i<amount;i++){

image = this[“image_”+i];
image.onRollOver = function() {

this.b = 100;
this.a = 70;

};
image.onRollOut = function() {
this.b = 50;
this.a= 35;
};

image.onEnterFrame = function(){
this._xscale += (this.b-this._xscale)/15;
this._yscale += (this.a-this._yscale)/15;
};
}

i have this function on all my thumbs…

I got a 50 x 35 Mc with a bitmap in it…
when you rollover i want it to scale to double its size, rollout it goes back to 50 x 35…

but this is not the case… the thumb scales weird. like only width is being scaled… then when rollec out it scales down even smaller than the original…
is there some i need to know about scaling?