I'm a big guy too. I have a Big Agnes down one that I use in cooler fall trips. They have some that are NOT mummy style. They dont have insulation in the bottom, instead a pocket for your pad. So they kinda perform like a quilt.
Another option is just use a cheap 40F bag, unzip and use like a quilt. This is what I do, as a lot of summer trips it's well above freezing.
But like others have said, a true quilt is the bomb... from what I hear. Enlightened Equipment seems highly recommended.
A huge, heavy ... yet cheap option, is the Coleman Big Basin big & tall 0-20F. Normally I wouldn't mention such a big & heavy bag but for us big guys any suggestion is valuable info. I use this bag with a BA Q-Core pad .... but the mules pack it. Quite a comfy bag.
Hunt'nFish