Is this a duck egg? Why is it so small?

[Raea]

So this morning I found my first egg in my ducks pen and I was so excited! My Pekins are only about 21 weeks or so. I read both 24 weeks and 7 months was the age they would start laying so since I didn't know which was correct, I figured I'd just wait to get them nesting boxes until I knew they were starting to lay. So is it possible they are already laying? I'm a little confused though because after my excitement passed, I realized the egg is very small compared to what I was expecting. At first I thought one of chickens somehow got in there and layed an egg, even though they're really good about going in their boxes. After scraping some of the gunk off the egg, you can see that it is white which none of my chickens lay. So I'm assuming this is my first duck egg? My females have been more dominating over the males lately, if that means something. Why is it about the same size as my chicken eggs though, I was told they'd be bigger? Will they possibly get bigger, since this was their first one (probably, since they free range all day so I'm unsure)? Also, is it as easy to train a duck to use nesting boxes as it is a chicken and should it be off the ground like a chickens too? Thanks for the help and I added a photo so you can see the duck egg I found on the left and my typical chicken egg on the right for comparison.

 

[Cherib603]

It's more or less a "practice" egg. Lol. Expect all different sizes and shell thickness as they crank up the egg factories!!!