But do we know the "ideal" funding values for the pipes and museum (and if we can, then why not use that)? It's only really "unfair" if we know that the pipes "deserve" a disproportionately larger multiplier. If the pipes deserve to be funded regardless of contributions (and they probably do), then the issue is using a system that could possibly fail to provide for them in the first place.
It's not a good way to allocate funds, but I don't think it's a slam dunk to say it multiplied a larger group's money more than it did a smaller group's.
Well it's also unfair if we assume large difference in the marginal utility of wealth -- for example to go to extremes, we might assume that a $10 contribution from a low-income individuals represents the same sacrifice as a $1,000,000 contribution from a high-income individual. If that were the case, a $100 contribution from a low-income individual represents 10x the utility of a $1,000,000 contribution from a high-income individual. So in that case the lead would pipes have both more contributors, and higher utility per contributor, than the art. So total utility would be maximized by giving more money to the lead pipes.
It's not a good way to allocate funds, but I don't think it's a slam dunk to say it multiplied a larger group's money more than it did a smaller group's.