iframe height and top issue

Hi All,

I have an iframe in my page.
I change its height from a button inside the iframe using javascript. for example:
parent.document.getElementById('iframe').style.height="100px";

The problem is that if the iframe height was 500px for example, and I changed it to 100px then the whole iframe is shrink down. I mean that the 100px are the top 100px but they are appear in the button of the iframe and there are 400px empty at the beginning. Any Idea how to change it?

Thanks in advance

Why not to use DIV`s?