What is the origin of the term "wedding clock"? Is it because they were given as wedding presents? Or were they purchased by newlyweds who couldn't yet afford a larger clock like a grandfather clock?

An example can be seen at the following link (it's the one on the far right):
http://www.floatingstonewoodworks.com/stickleyaccents.htm

I've never heard of a wedding clock, but i have heard of anniversary clocks.. and i suspect they are one and the same..

they only needed to be wound once a year, (on say your wedding aniversary).
now that most have quartz electric motors, the name is a misnomer.

Notice what it looked like? It grows on you. After a couple of years it's a father clock and after about 25 or so a grandfather clock.