It really just depends on whether the page is height-limited or width-limited. In your screenshot, the page is height limited (as you can see black space to the left of the left page). When you reduce the height of the second page, it's no longer height limited, so it expands the width as much as possible and becomes width-limited. That's why the two pages appear different as a different amount of scaling has been applied to each.
As a side note, if you turn the page scaling to "Fit Width" for example, the two pages would have the same scaling, but the tops and bottoms of the left page would be cut off.
As a side note, if you turn the page scaling to "Fit Width" for example, the two pages would have the same scaling, but the tops and bottoms of the left page would be cut off.