I won't comment on the light vs heavy thing, but as far as judging yardage, I have a solution that will improve this for most people.

The human brain is a wonderful thing if it is given some repetition. People don't judge yardage well, because people don't practice judging yardage or don't practice judging it the right way. If you repeatedly take a target at an unknown distance, guess the yardage, shoot, then pace off the actual yardage, your estimating abilities will greatly improve.

I see lots of hunters practicing many hours with their shooting and tuning their bows to a fine degree. Some even shoot some 3D, which will help a little with yardage judgement, but they're concentrating primarily on score. Seldom do I see anyone concentrating just on yardage. Probably one of the best ways is to get out and do some stump shooting. Concentrate primarily on judging unknown distances and then pace off the actual distance so your brain can record both. Put 20 minutes in a couple times a week practicing this, and you will see vast improvement.

There is a lot of technology available to modern day bowhunters that makes it easier for us to take deer. I think this lends to the attitude that we don't have to practice as much, which may be true to some degree. However, if you want to be really good at this hunting thing, then intense practice will give you your best chance at success.