Right but we're not talking about energy of the homogenous object. The brain or heart or lungs of fat person won't be that much heavier, but the fat around it might work as cushion to spread the spike of deceleration over longer period (less G's over longer period of time basically). Then again, the fat on the other (back) side of the person might compress squishy stuff inside more so I dunno how well that can be simulated in a dummy