I saw Aretha Franklin on TV once, I think with Oprah, a few years back. She was asked if she found there to be an optimal weight for her singing (in regards to being out of breath, belting out songs harder or being able to hit higher/lower notes etc...) and Aretha said:
"I find about 160lbs to be the best for me. I felt the best and sang the best at that weight."
Aretha hasn't been 160lbs since the mid-late 70's if not longer than that. And when she said that she was pushing at least 300lbs if not 400lbs.
Killer singer at whatever weight she's at, for what it matters.