It depends on which version of Chun you’re talking about. Chalk it up to sprite style changes if you want, but I think that SF3 Chun Li’s thighs got so much bigger on purpose. She’s older and stronger. With that much muscle in the lege, and her height, 150 seems about right.
Basically, an arbitrary number. SF characters’ heights and weights are totally crazy already (ahem T.Hawk). A 5’9" Chinese policewoman is already quite fishy.
The only numbers I can suggest are 48 and 68 kg. 48 was on a sticker that came with CVG back in the HF days, so it’s obviously not very likely. 68 comes from Juni’s scannings in SFA3. All other characters are given numbers that match with their height and weight. Chun-Li’s height also matches, and so it’s quite likely that the weight is also the real one.