My Big Fat Earthworm

I was in the garden this weekend fluffing up the soil and adding compost when I unearthed this bad boy. Maybe it doesn't seem huge in the photo, but let me tell you this worm was the biggest, thickest one I've ever seen in the garden.  When I first spotted it, one segment of the body emerged from the dirt, and I thought it was a massive grub.

Is this just an overweight earthworm?  Are there any types of earthworms that are bad for the garden?