Since the version 2.1 you can choose whether to keep the total height of the article layout or not when scaling a headline vertically on the Pages main tab.

This is a personal preference. The default is not to keep the total height.

Quick select

To change quickly the status of the 'keep the total height..' function, right-click the Frame tool, and then tick or untick the Keep Total Height When Scaling Headline.

Select in Preferences

Alternatively, on the Preferences of the Pages main tab (Edit > Preferences), click Extra tab, click Split, and then tick or untick the Keep Total Height When Scaling Headline option.