Are you sure it's the greens? Maybe it's the stuff you put with your greens. Just for an example, if you put a really tangy/zesty dressing on your greens in your salad, I could see how that might make you thirsty. I don't know about the smoothie though -- what else do you put in it?
hmmm...the only thing the two have in common is the greens...
i use about a tablespoon of balsamic vinaigrette and feta cheese dressing mixed on my salad as well as nuts and cranberries. This is completely different from the smoothie where I use yogurt, some fruits, maybe chocolate syrup