Yeah, I thought about the hollowness problem as well
This was my thought on it, when I made the calc: Grun's body however is curved in the dimension we cannot measure with this pictures. Also the massive EMP generators poking out of back cannot be measured as well. The weight which would be lost here, will be used to account for hollow spaces inside Grun.
However it is understandable that it might not be enough to account for possible hollowness inside him. What do you think would be a good reducement factor considering we already lose a bit of weight because of abovementioned reasons. -30%, -40%, -50%?