Problem: different number of posts per page VS. loop and Infinite Scroll

Hey,

I have some problems with the number of posts per page, the loop, and the plugin “Infinite Scroll”.
Infinite Scroll is loading one page after another on one singel page – without reload – just jQuery.
My problem is, that i simply want on the first page less posts (13) than on the following pages (18 – global standard is also 18). What i get with the following code is the correct number of posts per page, but also 5 missing posts on page 1 (the last 5 posts >> 13 + 5 = 18).

if(!$wp_query) global $wp_query;
if (is_category(4)){
	if ($current_page == 1){
		$posts = 13;
	} else {
		$posts = 18;
	}
}
query_posts( array_merge( array( 'posts_per_page' => $posts ) , $wp_query->query ) );

What can i do to get the correct number AND the correct position of the pagebreak? Maybe there is a way to define the number of posts per page for each page before loading?

I hope you can help (and sorry for my bad english)

Cheers