...yeah that's screwy. Either Mikasa is pretty much all muscle under there or her weigh here is simply incorrect. Girls normally that size probably should be somewhere maybe between 125 to 140 lbs assuming they are particularly slim or skinny.
Well, remember Mikasa was always pretty strong and I think it means she's pretty muscled as well. But I think she's normal weighing, I mean I can only relate it in kg and cm and 68 kg for 170 cm is okay in my opinion.
...but she's also 15 as well. Sorry, but I don't buy it. If it happened to be a male probably, but a female? Again, she would really have to be ripped so much so that there is no way she could have the feminine figure she does.