the reason the avrage sith apprentice could take down a padawan or avrage jedi for that matter is due to the fact
to become a sith apprentice you got to be better than avarage
you have to kill to get the job
This is totally right. However, early in a force-users development the dark side is much easier than keeping to the light. The Dark Side is permissive, and doesn't even really penalize you much for straying from time to time. One a force-user reaches their prime, that aspect of the darkside no longer makes progression any easier.
Essentially it seems that developing the discipline and focus of the light side makes it take longer to get all the basics down, as you're dividing your efforts, while the dark side actually makes getting up to speed easier. Once they've got all the basics down and their power has matured, honing their abilities any further is just as hard for both.
Any disparity in apparent power between advanced light and dark force-users is due to the fact that darksiders tend to die early if they aren't exceptionally strong, while lightsiders can live a long and healthy life even if they barely have any power at all. So their top people are about the same, but the Jedi have a lot of weaker members filling out the ranks as well that their Sith counterparts would have horribly murdered.