No. They would pay a share equal to their usage. Outsized users would pay the same per GB as smaller users, and would pay more because they use more.

ISPs who were zero rating partner content now have to either zero rate the entire category (therefore putting spotify and itunes and play music in the same bucket), or not zero rate. This appears to be the "subsidization" argument, and it's nonsense, since the ISP could just zero rate nothing.