To put it very simply, "low" and "high" dosage pills just depend on the level of different hormones that are in the pill.
If you are on a low dosage pill, then your doctor knew that your body would react better to a lower dose of hormones. If you are light-weight, then many "higher" dosages would give you too many hormones for your weight, thus giving you unnecessary side-effects.
Both types of pills offer the same amount of protection against pregnancy, so don't worry. Different people just react differently to the doses of hormones