My grandmother had a secret for light, fluffy matzoh balls. She passed it on to my mother, who passed it on to me. Two secrets, actually. (And not such big secrets, because they appear in lots of recipes.)
1) Use seltzer for the liquid.
2) Keep the lid on the pot until the balls are done cooking.
In theory, the carbonation in the seltzer creates air pockets that lighten up the dumplings (which is pretty much what matzoh balls are). The steam from the simmering soup does the same thing.
Personally, I like them dense enough that you feel them sitting in the pit of your stomach for a week. Best of all, if you have leftovers, chill them, cut them into slices, and fry them in oil. It's sort of a Jewish polenta.