Well, first of all someone that is SO heavy won't try doing the kind of things Wii Fit sugests you to do. Someone over 300lbs might get its knees injured if they try some spinning or leaning or something like that.
Woah, looking at those weight comparison charts reminds me when I was near 90Kg. I was so fkn fat. I still feel fat at my current weight (72Kg./1.86m.)