Animation in ImageReady

I'm hoping someone has a word or two of advice...

I'm having problems suddenly with creating animated icons. I open up an .mpg in ImageReady, like usual, click on Select Range and Every 2nd Frame. But even though I have under 30 frames of animation (sometimes under 20 - or even 15!) my icon saves at a ridiculously high file size. I had 27 frames in one icon and it saved at 125kb! Another icon has only 17 frames and it's file size is 50.0. What is the problem?