all vocalist pop girls are kinda overweight by our culture standards.
Adele, Kelly, Demi,Xtina ... all those singers serve vocals. And lately Lady Gaga got a lil weight and her voice started to sound strong on her jazz albums. This is also happening to Katy, which we saw during the live performance of Rise.

Do the belly and vocal chords have any correlation with each other, is there science behind this observation?

No shade yall, only civil discussion plz

No, it's more to do with your diaphragm (if you're singing with a good technique) because that's where you support from.

The fatter the belly, the smaller the penis