"Girls, why is it ok for girls to discriminate against short men, but not ok for men to discriminate against heavy women?"
Short answer - Political Correctness.
Classic feminism fought for legal equality way back in the day. Now, modern feminists, that have all grown up under full legal equality, continue to use the legal term "discrimination" in order to describe any kind of social or personal preferences that work against women (only), under the idea that this "discrimination" against women is somehow equivalent to legal discrimination.
The obviousness in this is brought to light by so many feminists responses to this type of question. When men face social/personal discrimination, then they are not victims of any kind. According to feminists, they are just the recipients of human preferences.
But when the EXACT same thing happens against women, then it is "discrimination" (a type of legal term), which must be taken seriously, and then fixed by all of society. This is what's at the root of the fat (body positivity) movement, and nearly every lame social justice article every time we log out of Yahoo. The "news" is always something that feminists and sjw's promote as "justice". Somehow it all follows the same leftist ideas that take --their-- politically correct issues as being as serious as any legal issue is. For everyone else (especially "white men"), your interests are considered your own personal/social problems for you to deal with on your own ... or just "man up" to.