If the thyroid were less active (producing less hormones), med dosage would have to be increased, not decreased. As gimel said, chances are, the doctor was reacting to a TSH level, not actual thyroid hormone levels.
We'd need to see the actual lab results, with reference ranges.
It may only mean that your doctor based his decision on your TSH level, which is not the right thing to do. Please post your thyroid test results and reference ranges shown on the lab report. Also, how were you feeling before the change? Any symptoms at that time?