Why is that more realistic than looking at real numbers? False premise in your argument. If 9/11 had had an even more destructive effect on 2001's GDP, than it did, government discretionary spending would have churned right along at the same levels despite the distorted ratio.
Brief economic blips in the GDP like the 9/11 dip or the 98-99 bubble distort the picture, yes. But relating spending with the GDP over the longer term, the blips disappear and you get a more accurate picture of how much of the economy is in the feds' hands.