Good explanation Steve regarding the theorie behing the necked cases of modern guns.
I do not know for sure, but am guessing there may be something else too.
If the case would have the same diameter of a modern small caliber bullet it would not hold much gunpowder to propel the bullet to extreme speeds. To get enough gunpowder in the case I think the case designers may have chosen a large diameter case on the bottom since a case diameter of the same diameter of the bullet would have to be extremely long, and therefore not very practical.
Another thing I read, found it very interesting, was a website of I believe a Swiss man who experimented with the earliest handguns. Basicly sticks with a touchhole. I remember that he reproduced such a weapon of relatively large caliber. But the diameter where the power was put was in fact smaller of diameter than the barrel itself.
When fired he noted a short sharp, smokeless powder like, bang. He also had a scientifical theory why the black powder. in the narrow section of this barrel, exploded much faster than in a regular barrel.
I can't remember what it was and I lost the website.
Aha! Just found it!
http://www.musketeer.ch/blackpowder/handgonne.html
This is very interesting reading.
This may indicate that the same amount of blackpowder in a narrower but longer collum may explode faster.
Maybe any pyrotechnic wizard among us that can explain if this is correct?