Android has a mysterious case of gigantism, and I'm not entirely certain why manufacturers keep feeling the need to have a bigger phone than the next guy. The size war (all male anatomical euphemisms aside) is on, and we're not sure when it's going to end. Take a look at these device charts for the three major Android manufacturers in (pretty much) chronological order of release:
High-end phones only. No QWERTY devices. Phones ordered by release date - more or less. Data from GSMArena.com.
Somewhat ironically, Motorola has shown the slowest rate of touchscreen device size-creep of any of the major manufacturers, having started out pretty big with what, at the time, was the gargantuan DROID X.