Compared to a buff orpington?? It takes about 3-4 coturnix eggs to equal one regular egg. Take your index finger and touch the tip to the knuckle of the thumb on the same hand, to make an "O". That's about how big the coturnix eggs are, give or take. They're perfect bite sized morsels... I'm in the middle of making pickled eggs right now!
The eggs I just hatched mine out of were bigger than that. They nearly equal my japanese bantam eggs. I almost fit serama eggs into the same plastic cartons the coturnix eggs came in. The smallest one probably would have fit but the average one was a little too big so I had to pad chicken cartons to hold them straight. I would say about 3 equal a standard large chicken egg.
I was shocked at the size especially after being used to button eggs. I took a pic next to a button egg but I dunno where it went. I should have taken one next to one of my bantam eggs. I guess I'll have to wait and see if they lay as big of eggs as they hatched out of.
Had the same experience with my Jumbo hatching eggs that I ordered. I already have the regular size coturnix and was familiar with the size of their eggs but WOW! The jumbo eggs were so huge that they didn't fit well in my quail egg turner cups in the incubator. They looked like they would fall out when the cups were tilted all the way to the side (they didn't). We still had to separate the turner trays so they weren't right next to each other, though, as the largest eggs would actually hit the edge of the tray next to them when tilted all the way to the side.