Since Akregator uses a KDE runtime environment, it is independent of many Gnome preferences, including UI font sizes. (Note that you can set browser fonts in Akregator by choosing Settings > Configure Akregator… > Appearance > Font Size.) You have to edit a configuration file, ~.kde/share/config/kdeglobals, to change the font size. For example, the following sets all UI fonts to 14:

[General] fixed=Monospace,14,-1,5,50,0,0,0,0,0 font=Sans Serif,14,-1,5,50,0,0,0,0,0 menuFont=Sans Serif,14,-1,5,50,0,0,0,0,0 taskbarFont=Sans Serif,14,-1,5,50,0,0,0,0,0 toolBarFont=Sans Serif,14,-1,5,50,0,0,0,0,0