This is a Bioware implant similar in effect to the cyberware [MuscleReplacement]. Each level grants 0.8 Body Index, +1 ST, +1 DX, and increases your weight by its WSR of 12. So if you're a ST 10 human implanting three levels of MuscleAugmentation, you start out weighing 150 lbs, and afterwards weigh (13*13/10 - 10*10/10)*12 = 82.8 extra pounds, for a net weight of 233 lbs. Your WSR is down under 14 now, too, so you get some extra nimbleness from a level of Negative Encumbrance.