Complex sentence: Consort Yang was known for having a full and fleshy figure, which was a much sought-after quality at the time, but must not be viewed in the modern context of someone who is extremely overweight.
Split into simpler sentences: Consort Yang was known for being slightly overweight, which was a much sought-after quality at the time. The term "obese", when used to describe Yang Guifei, must not be viewed in its modern context of someone who is extremely overweight.