Tree Density with large Overgrown tree scaling has been discussed in the recent MSFS Developer Q&A Twitch Series: Top Community Question 27/01/21
see Time Stamp 48:25 Overgrown trees issue
The MSFS Feedback snapshot has Tree Density listed as Ongoing Improvements, it's on the backlog by Asobo to look at down the track likely later this year.
The current Tree situation given is that the Trees were made in MSFS much wider than they are in real life. The benefit of doing this in MSFS was to provide a far broader Tree coverage area, allowing for a smaller number of trees to be used in order to cover a greater area of Tree coverage.
Basically if you put accurately model Trees into MSFS then they wouldn't cover the correct amount of area per coverage distance, so you would need millions more of them. Having millions more means suffering from overall Sim Performance issues.
Once DirectX12 optimization has been implemented due mid year. This should give Asobo some working head room for performance increases meaning that Asobo could potentially be able to shrink down the width of the Trees thus increasing the number of trees and species available, leading to some more realistic Tree height appearances.