Change posts_per_page inside loop?

Hi there,

I’ve got a problem I can’t seem to solve…
On a page displaying posts from a custom post type called ‘news-item’, I want to change the number of posts displayed on page 2 and further. This because I want the newest (first) post displayed bigger then the rest, but therefore the layout/grid has to change too. So for example:

On Page 1:
1 Large post
4 normal posts
—————
5 total

On Page 2:
6 normal posts
—————
6 total

The part that the first post has to be bigger on page 1 works, but I can’t seem to change/set/update the posts_per_page parameter inside the loop in order to add it with 1. Is that even possible without making a second wp_query?

I tried it with:
set_query_var( 'posts_per_page', 4 );
and:
$args -> set( 'posts_per_page', 4 );
but that’s not working and messing up my pagination.

My code: https://pastebin.com/hR6K4Wtj

ANY help is welcome!
Many thanks!