Not entirely true, actually:
2009 Australian Open - $1.75 million USD for the winner (men's singles)
2009 US Open - $1.6 million USD.
2009 Wimbledon - $1.35 million USD.
2009 Roland Garros - $1.35 million USD.
It's quite clear to see the Australian Open actually has the
biggest pot for the winner, not the smallest as you claim.
And there is one thing you forgot: the amount of price money has taken a drastic rise in the recent years.
For example here is the prize money for the men's singles champion from 2003 onwards (the year Federer won his first title at SW19)
2003 - 915.000 USD
2004 - 960.000 USD
2005 - 1.000.000 USD
2006 - 1.045.000 USD
2007 - 1.115.000 USD
2008 - 1.195.000 USD
2009 - 1.355.000 USD
Federer won all of those save it for 2008. On average he has colleceted 1.065.000 USD from winning each Wimbledon title.
Same thing for each tournament, for example The US Open had 1.500.000 USD for the winner in 2008; a year later it was 1.600.000 USD. In 2006 it was "only" 1.200.000 USD.
I am nitpicking I know, but I am bored out of my mind