|MadSci Network: Genetics|
You have discovered one of the great mysteries of biology. Nobody knows why different species have different amounts of DNA. On a quick search of the Web I found this slide.
It is nothing to do with complexity, or life history, or anything like that. It is just a puzzle.
One thing you might try and do is see whether the amount of DNA you extracted from the different vegetables is roughly the same as other people get, but I think it will be difficult to find out how much the different vegetables contain.
Try the links in the MadSci Library for more information on Genetics.