Big Eggs?

We have a mixed flock and not all their eggs are the same size. The largest ones and they are LARGE, are laid by our Red Sex Link girls and the smallest by the Speckled Sussex (although they are a decent size too). Everyone else (Plymouth Barred Rock, Easter Egger, Rhode Island Red, Black Australorp) fall somewhere in the middle of the other 2 breeds.

I think what you're seeing is normal, Every once in a while one of the girls will lay an especially big egg that turns out to be a double yolker but then they go back to laying their 'normal' size eggs.

 

Probably a double yolker.
Did they just start laying?

 

Here's a quote from the Common egg quality problems" article:

5. Double or multiple yolk eggs
These are more common with your pullets just coming into lay. This happens when more than one yolk matures and gets released at the same time. As many as 9 yolks have been found in a single egg.

You can read the entire article here (pretty interesting stuff and good for future reference):
https://www.backyardchickens.com/a/common-egg-quality-problems

Hope that helps!