Just some numbers, based on my hardware:
Lion OS X install on disk: 7GB
iOS 5.0.1 install: 1.7GB
My MacBook's RAM: 8GB
iPod Touch: 512KB
MB's disk size: 100GB
iPod Touch: 8GB
So: Maybe they removed 80% of the API in general because the iPhone/iPod touch is only 10-20% of a normal computer? Something had to go.