Sam is half-right by saying that taxes are garnered from the rich. With GST, the bulk of cashflow from taxation comes from consumption taxation within the country and not class-centered.
The rich (i.e. myself) can choose when, where, what and how to spend my money should I choose to. This can either effect capital flight (buying a multi-million dollar home overseas, spend on cookery courses in France) or tax revenue (buying a GCB, D9,10 condo in SG or learning how to cook local favourites). If I were less ethical, I could further lower my personal tax exposure in SG by stating these as expenditure due to business or even pay for my condo via CPF top-ups, which is an outright deduction from my P&L or non-taxable, and IRAS will not even bat an eyelid so long as my tax returns grow YOY.
Put is simply, I can pose as a frugal business man, knowing that I my company will provide for me and my spoilt rotten children.
However, when you look at the middle class, they are effectively the "squeeze class", most of their spending is non-discretionary, i.e- they have no choice but to spend. These are garnered from day-to-day consumables (food, utilities, transport, communications, ERP etc), and thus give the best level of certainty of cash flow in terms of tax revenue.
Therefore, it is not specifically true that the rich pay the bulk of taxes, but rather from the fact that they can (if they want to) effect larger tax income from discretionary spending.