Gridview size is changing after refresh page

I'm almost production-ready! Which means I'm getting down to the picky stuff...

My gridview comes up intially a certain size. When the page the grid is on is refreshed, either because of F5 in IE or because I do some processing and want to repopulate the grid with less rows, it is not the same size. It is actually a better size.

So I would like to know how this can be controlled and why it's not the same after a refresh or page load, and how can I have it be the size I want (it's more "squeezed" the second time without overflowing and wrapping in individual columns).

Thanks.