I personally don't think being worthy of Mjolnir gives you Thor's stats. It gives you his power of the storm and all of the associated Mjolnir tricks and whatnot, but not Thor's physical stats stacked on top of your own. To that end, Bill was probably remaking over the power of Mjonir itself.
If Bill did get Thor's stats added on top of his, that would likewise mean he was about twice as strong as him with Mjolnir in his possession. And considering Stormbreaker was made to be equal to Mjolnir in terms of function and power, it would likewise mean Bill is 2x more powerful/stronger than Thor is...which we know is anything but true.