My app is using the largest system font available to my knowledge, FONT_NUMBER_THAI_HOT, yet it still doesn't come close to the larger font size shown in the weather widget on the fenix 5. I would think they are using system fonts for the default widgets, but could they be importing fonts with larger sizes?
It might be using a font that's not exposed to CIQ. If you'd like a larger font , you can always make your own custom font. Details on how to do that are in the programmer's guide