How the hell do you get from here to here?
I don't mind explaining.
By floating idea that only absolute value of contribution matters you have to accept that only absolute value of anything should be considered.
This way someone donating 1 million will always be considered more generous than someone donating 1$.
I don't necessary agree with this idea, but in order for anyone proposing such idea to stay consistent they have to extend it to other areas - such as taxation.
That is - if bottom 20% don't contribute in absolute numbers much to tax revenue, it doesn't matter (as in not related to notion of fairness) if they don't pay anything at all. So why do we even bother taxing them?
So catch was - if you say that % of your income contributed to charity does not enter "fairness" or "generosity" consideration, you also have to apply it to other areas of life - like tax system.