Okay, I just went and reread chapter one to see, and I disagree. She's barely changed.
By page two, she was sleeping like a kid. Even in chapter one, the guy pulls out this very same ice cream and Ruu has basically the same reaction of instant mollification.
The only difference over time is that, aside from being made to feel comfortable and sleepy at every turn, now she specifically cares about the well-being of the guy. Which is understandable since she's been living with him for a while and enjoying his company.
It's true though that the page count has decreased...I didn't remember that it wasn't always a single page. But even within the longer chapters, it was still pretty much one page per gag; it's just a matter of how many we get at once.