The Treasury Department reported on Oct. 15 that the deficit in fiscal 2010, which ended Sept. 30, was $1.294 trillion. That’s less than FY 2009’s $1.416 trillion, but it’s still really really big. Why is it so big, though? Is it because of all that stimulus and bailout spending? Or is something else going on?

To find out, I created a fantasy world. I figured out how fast federal spending and revenue grew over the last business cycle, from 2000 through 2007, and calculated where we’d be today if those growth rates had continued through 2010. I was originally motivated to do this for a commentary that’s supposed to air tomorrow night on Nightly Business Report. But I’m thinking there’s not a huge overlap between Felix Salmon readers and Nightly Business Report viewers, so I’ll go ahead and share what I learned.

In my no-financial-crisis, no-bailout, no-recession, no-stimulus scenario, spending kept growing at 6.22% a year, and revenue kept growing at 3.45%. You can see from the difference between the two numbers that this was an unsustainable path. But it clearly could have been sustained for a few more years.

Where would it have left us in fiscal 2010? With $2.843 trillion in federal revenue and $3.270 trillion in spending, leaving a deficit of $427 billion. The actual revenue and spending totals for 2010 were $2.162 trillion and $3.456 trillion. So spending was $186 billion higher than if we’d stuck to the trend, and revenue was $681 billion lower. In other words, the giant deficit is mainly the result of the collapse in tax receipts brought on by the recession, not the increase in spending. Nice to know, huh?