it's somewhat about how quickly the simple sugars hit the blood stream which affects diabetics drastically. the calories from complex sugars like starch- some will be used in digestion and the rest are in the simple sugars that are more slowly released into the blood stream.
either way, the body is getting an influx of simple sugars that it has to metabolize in some fashion.
the other complex sugar compounds are fiber - very hard to digest - in fact, we use microorganisms to digest them for us... and much just passes through the GI tract.