OK, so I created a test thread called "Experimental thread ignore" and I filled it with VERY VERY long spoilers that included images, and I still got to complete 25 posts before it went to another page. I then went and looked at several other threads and they all started the second page on the 26th post. So I am beginning to think that the software allows 25 posts to a page, no matter what the content of those posts...
So, I guess, what we really have is a situation where using a spoiler will reduce the scrolling for an individual page, but the number of posts is what determines a new page, not the amount of data the posts include.