In Pages 5, why are page thumbnails different sizes?

So I'm using 5. And I must say I think it's brilliantly done after a long period of ******** about it. I guess I'm not asking of it anything it doesn't provide and what it does it does very well - though I'm puzzled by the behavior of selecting text via the keyboard.

Anyhow, that's not my question. I'm working with a long text document. I notice that as I work the page thumbnails become different sizes. It's subtle but it's clearly intentional. I reckon it's trying to tell me something but I don't know what. Haven't been able to find any info in the manual and was wondering if any of you clever people here might know. Thanks.