I'm not sure about that. Sirius was a good enough wizard to notice someone else casting the spell that killed the muggles and it would rely on good timing from Wormtail to change into rat form.
Its noticable (sp) that Wormtail cuts off his finger and then cuts his hand off for Voldemort - rats will chew off their feet when caught in traps to ensure their escape. Therefore, I believe that Wormtail did the magic himself and reacted in the only way he could think of. He knew he would never defeat Sirius in a straight duel so had to be sneaky to save his skin.
Perhaps when people began to follow Voldemort his power was sort of shared with his followers. I mean he is very powerful and it says somewhere that the death eaters were nearly as powerful. Maybe his close allies have their power boosted somehow by their association with LV.