6'1 and 200 lbs I'd say not on a "heavy" side. Personally I wonder if you should of go with smaller Western side. "General" rule is English size = Western size + 2, however for English saddle size it also depends on the length of the leg. For example, I rode in 15 Western, however both of my English saddles are 18 (I have long legs). My guess would be 18, may be 19 should be OK size for you (although it's a guess of course :) ).
BTW, here is the calculator for the seat size you need: English Saddle Seat Size Calculator