I was considered underweight when I started. Was low in muscle and low in fat although definitely had a little around the mid section (carb belly). The first thing that happened post-primal was I lost the carb belly. Next my testosterone started increasing by a very significant amount. Now I'm packing on muscle mass with no visible increase in body fat. It's pretty nice no longer being a "hard-gainer".
Really overall total weight isn't anywhere near important as where the weight rests on your frame.
"You can demonstrate the purpose and limits of human digestion with a simple experiment: eat a steak with some whole corn kernels, and see what comes out the other end. It won’t be the steak."