There are details under spoiler.
Spoiler
(http://i.imgur.com/kyDg3wc.png)
AGS Editor shows that font has 10pt in both cases.
In General Settings check if "Fonts designed for 640x480" is set to "true".
PS. That means "designed for the resolution you are using" in general, as opposed to "designed for lowest possible resolution and must be scaled up if resolution is higher".