FWIW, here is my two cents;
First, it doesn't matter WHY you feel less pain/strain...you do, and that's what matters. If you can adapt to your new shooting style and hit within your comfort zone, continue to do so.
Second, that being said, just because you can pull more weight pain-free doesn't mean you SHOULD. Heavier bows will always put more wear & tear and strain on the body, including muscles, tendons and joints. A long as you are hunting deer-sized and under game, my advice would be to continue to use the light bow and don't push your luck. What feels "fine" today might not next month...if you get my drift.
Of course it's your body and your choice, just thought I'd throw that out there.