Sometimes discrimination is already rather hard to prove, very often not even possible.
Then, there are already people using those protected classes to behave like little shits (those people are everywhere, but of course also here).

And now they try to add 75% if all people to the "protected class"?
This will undermine every effort of protection against discrimination, because it is a perfect argument for discrimination, because it makes the whole law absurd.
And, that's the thing, people generalise.

"An absurd addition to the law? The whole law is stupid and we don't need to adhere to" will be used as an excuse to discriminate, not many follow (perceived) stupid laws.

And discrimination is hard to prove (which, unfortunately, is the better way, otherwise you could just run around and sue everybody for discrimination, would it be easy to prove).

So no, adding something like "size discrimination" to the protected classes would be more than dumb.

Disclaimer:
It's hard to fully formulate the thoughts without writing a small book, so most probably something came out wrong and I apologise already.
Additionally: I go out from the Austrian version of "protected class" because I don't know how the US system works. There are probably quite some differences, so sorry for the confusion!