Very small font in Fenix 6 PRO (47mm) - not true that it is 18% bigger but 30% less readable compare to F3

Are you talking mainly about the Widget Glances screens (which crams a bunch of widget views into a single table of rows) that is on the F6 but not the F3? Is it any better if you click into the widget itself and see the bigger datafields on their own?

To answer your question "Is there any way to increase font size..." :

NO. (Fonts are bitmap fonts with fixed size, which cannot be scaled)

Bigger display resolution does not necessarily mean it is more readable:

Fenix 3 resolution is 218x218 pixels, fenix 6 pro is 260x260. That is 19% higher rsolution.
BUT display size of fenix 3 is 30.4mm diameter and fenix 6 is 33.02mm, which means it is just less than 9% bigger.
It also means fenix 6 has higher pixel density (about 10% more pixels per inch).

10% higher pixel density means that the same font with same font size will be 10% smaller on fenix 6.

If the default watch faces you can choose from do not give you what will be easier to read, there are hundreds of watch faces you can choose from Connect IQ using Garmin Express or the Connect IQ APP.

Try submitting a feature request to Garmin for a way to zoom in on a view.

https://www.garmin.com/en-US/forms/ideas/

Not sure how that is going to help as soon as you leave the watch face screen