You can increase the tensile strength of fiber by quite a bit without adding much weight depending on the material. Being that she's mutated her muscles are probably laced with other things that increase their strength. Bone density would be where the added weight would kick in. If it really bothers you, assume 15% of body weight is bone and guess she might weigh an extra 90lbs/40kg.
The whole weight thing is kind of a handwave in super hero things. When was the last time someone asked the weight of Captain America or Superman? Or if you want to risk your life Supergirl.