This is the default font size for text on this site, which is defined in the CSS style sheet.
The larger text on all the pages of this site was hardcoded when content was created, by using a Size = 'Small' in the text editor which overrides the site's style sheet.
The correct way to handle this is to increase the default Size in the site's style sheet, and then do not override the size in the text editor when creating content. Once the style sheet is updated, it is a global change throughout the whole site that will increase the font size in all locations.