A lot of these answers seem to be complicating things more than they need to.
Now, maybe this isn't a purely ethical answer, but it is by far the simplest
Lets say you're in a group of five friends. they have $2, $4, $5, $9, and $80.
All of you are out in town and need to buy something to eat. A pizza. Let's say it costs $20.
Now, you could say that everyone should just pitch in $4. that might be fair in a sense, except hold on, one of you doesn't have that much money. So you either need to buy half a pizza for $10 (with each person contributing $2), which isn't nearly enough for all of you, or accept that there needs to be some level of unequal (in quantity) contribution.
Ok, option 2 is everyone contributes 20% of what they have. ($0.40, $0.80, $1.00, $1.80, and $16 (for $20 total). This works out rather fairly, if pizza is the only expense everyone had, but wait. Everyone needs to pay for the bus home too. that costs $1.50 for everyone except the guy with $80, who's taking a taxi home since it's faster. He's paying $4.
The problem now is that even though we're spreading the pizza cost around as an even %, the guy with $2, is spending 95% of his money on this day out with friends, and the guy with $4 is spending 58% of his money on this trip, while the guy with $80 is spending only 25% of his money. So this doesn't seem very fair to most of them either (although some disagree, probably like the guy with $80)
So option C is to charge different percentages depending on how much money they all have spare. let's say each of them contribute 22% of the money they have -free-. The guy with $2 has $0.50 free, contributes only $0.11, the guy with $4 has $2.50 free, so he contributes $0.55, meanwhile the guy with $80 has $76 free, so he contributes $16.72 instead of $16 in the earlier plan.
This last option leaves the group paying an increasing percentage based on their amount of wealth, with the $2 guy contributing 5.5% of his money, while the guy with $4 contributes 14%, and the guy with $80 contributes 21%, but leaves each with a more similar percentage of their wealth free to spend on whatever else they want, although still somewhat weighted towards the people with more money ($2 guy has $0.39 free (19.5%) while $4 guy has $1.95 free (49%), and the guy with $80 has 59.28 free (74%).
This last option is the way most real-world countries apply their taxation, the parallels being pizza == taxation, while the cab ride home is other unavoidable living costs, which as in the example, increase with wealth, but much slower than income does (lowest tier incomes may be spending 80% of income on rent alone for example, while highest tier spends less than 20% on housing/rent)
So is it ethical to tax different people disproportionate quantities? It is when you consider the alternatives, either providing insufficient services to everyone, or all but removing low income people's ability to spend money on anything except the basic essentials of life such as housing and food (and perhaps even those).
Disproportionate taxation, like many things in life, is perhaps unfair, but it is less unfair and immoral than the alternatives, which in a sense, makes it the ethical choice.