Disproportionate would be more than 100%. As you said: the consumers are willing to pay a certain amount. A share of that amount can never be "disproportionate". Are you demanding the same humbleness of yourself when you negotiate your salary?
It is disproportionate given Pandora, after all is said and done, is yet to turn a profit. If Pandora was making millions or billions in income, its easier to argue they ought to pay a larger share to the musicians.
No one's arguing they should pay a larger share - just that they shouldn't pay less than they are currently paying.
And yet no one has explained why they shouldn't pay less than they are currently paying, when their competition pays far less.
Really it is a pretty horrible business to be in. You are totally at the mercy of the publishers, should you be really successful at customer acquisition and monetizing then you will be rewarded with higher royalties.
As it stands I don't see how it would be possible for Pandora to ever do better than small profits.
How do we know what consumers are willing to pay? How do we know this is a fixed number and can't be altered by other factors?
Not when you have plenty of other avenues to get revenue from and cutting off this specific one own't have much impact on you.