First, only certain regions get a red region marker on the map. "Riding areas" get a red marker. Or cities with trails and NO children riding areas.
When a region is added, the marker will be large until the backend cron script that calculates the region sizes runs once a week.
Regions are given a global ranking score using the following formula. Where "t" is all the trails within that region.
((AVG(t.rating) / 2) + (SUM(t.votes) / 10) + (SUM(t.faved) / 10) + (SUM(t.ridden) / 100) + (SUM(t.total_checkins) / 1000) + (t.views / 1000000))
The regions with map markers are ranked per-state (or country) based on the above global ranking popularity. If there are more than 10 markers in a state, then they are then grouped into 3 sizes using that popularity.
Size 1 = top 30%
Size 2 = middle 40%
Size 3 = bottom 30%
If a region has less than 5 trails, it gets a 4th smallest size.
Recently this was changed for some activity types so there is now only 2 sizes, the largest and smallest (5 trails).
Only biking, hiking & trail running have the 4 sizes now.