sure, it's relatively simple.
Elaborate por favor.
the less money a person makes the higher percentage of their money they have to spend to make it through the year.
a person making 25,000 dollars a year is almost certainly not going to be putting money into a savings account, for example, or investing it in the stock market. Almost everything they make they'll have to spend, be it in the form of rent, groceries, car payments, repairs, clothes, etc. State depending, of course, due to different tax laws and exemptions and what not, all of this is going to be taxed through sales tax.
now of course it's not that simple. state by state, tax codes are very different, blah blah blah. with regard to illegal immigrants, it's impossible to know just what percentage they're sending back to their families in their original countries, if any at all. still, in the grand scheme of things, a lower class person is almost certainly going to be taxed on a higher percentage of their money than a person in the upper class.