That's hearsay. Besides, fighters often exaggerate their own weight and the weight of their opponent. Shevchenko has obvious reason to grossly inflate Nunes' weight -- it would make her performance against Nunes appear even more impressive if Nunes outweighed her by 30-40 lbs instead of 10-20 lbs. That is why she exaggerated Nunes' weight.
In the photo below, it was verified that Nunes weighed 156 lbs fully hydrated. She has approximately 30% body fat at that weight.
At 180 lbs, Nunes would have 40% body fat. So if Nunes really walks around at 180 lbs, it should be easy for you to find photos of her with 40% body fat.