Heavy != fat/overweight
And that kind of proves my point and not yours (well more his)!
How can it be then discriminatory?
Probably because it hasn't been tested. Obesity is self inflicted but beyond a certain point, it is pretty much a disability due to health complications, joint problems etc is it not? I'm not saying being fat is a disability in itself.
It has been test in law several times and it can't be demonstrated as such bar in a few individual cases. That is unless you want to apply the characteristics from a member of a group onto the whole group! And that would be ...