I discovered the problem! Posting here in case other people run into the same thing.

Feeling like it was just a problem beyond me, I threw out my old map and started again. While making the new map with a new strategy, I experimented with the Default Scale in Drawing Properties...which I'd used before...and this time it worked! It worked exactly as it should. Big forest, small forest, no problems. None.

I'd experimented a lot with the default scale in the previous, problematic map, and I sheepishly concluded that I have obviously changed some combination of settings I didn't understand and gotten myself into trouble. But now the issue was solved, so I start happily mapping away.

Then I discovered the problem. Every once in a while, when switching between two different Default Scales...it stops working. The new scale doesn't "take" and the symbols come out the same size as the old scale and when I then monkey with it to get it down, I get the results you saw above. Sparse forests for no reason.

If I quit the app and reload the map, everything goes back to normal. I think it *may* be related to switching apps during a session? Not sure. I haven't robustly tried to duplicate it, but it crops up regularly when switching scales and I can reliable fix it just by quitting and starting again.

Thanks again for you help, hopefully others with this problem in future will see this thread and be helped.

I'll post my final map when it's done.