Im in the uk, I would certainly say 50% isnt that big an over estimation...
For a starters if you earn over 150k a year you pay 50% straight up in income tax...
Even for average earners (take a 25k figure) you are looking at 20% basic rate, a further 17.5% vat (soon to raise) on most of your spend, you are looking at council taxes most likely around £1300+ a further 5%, you are probably going to be spending a further £2000 a year on petrol close to £1000 (4% of annual income)of which is fuel duty (VAT is extra on top of that). already its getting perilously close to 50% of your money going to the government, take into account the extra that transport taxation costs add to the price of goods you pay, the affects of corporation taxes, business rates etc adding to the price of goods you pay, It all adds up fast its all money transferring from the tax payer to the government...
UK Gross domestic product for 2009 was somewhere around 2 trillion. Treasury receipts around 878 billion. Not a million miles of 50% at all, BUT thats only central government that doesnt include local council taxes, rates, charges etc etc. Thats a further 45 billion approximately. Taking us to 923 billion, really not so far off that 50% of GDP...
Take into account that business's only pay tax on profits, and profit levels have been down across the board for a few years. A company treading water to survive is skewing the gdp \ tax receipts ratio in favour of the GDP.