I don't believe this counts as packet editing. I see no packet editor window. o_O
Sadly, I don't think that changing that would allow you to decide your weight. Not without normal (food/hot spring) procedures, that is. I would have to imagine that weight and muscle are server sided. Simply because it deals with your actual character. Now, if you got an actual PE then MAYBE you could send food packets to get what you want, but PEing is not something you can do just anytime.