The OS 61 FX weighs 700 grams, muffler included, the 91 fx weighs 770 grams, muffler included, the Magnum 91 weighs 725 grams, muffler included.
So you see the difference between a 61 and a 91 is very small, but power is way bigger in the 91.

If you want a really powerful and light weight 91 then get the Magnum 91 and fit a Tower 75 muffler to it and you are down to 680 grams. Yes, that is lighter than a normal 61 but far more power.