It's all about genetics really... None of my bantams EVER come close to laying as big as a leghorn... unless the bantam is an older hen and the leghorn is barely started laying pullet eggs.
Most
my bantam eggs start around 20-25 grams and top out at
45 g. Leghorn and other large fowl started out around 35- 40 grams and were upwards of 65-70 grams in year 1... getting slightly larger after that even. I used to obsessively write the hens' name, date, and weight on every egg collected. My EE eggs at year 3 are 60 grams.
@AltonaAcres, I don't doubt you.. but did you happen to weigh them? Or can you tell us where your Seabright eggs top out in respect to weight please? I'm curios as I bred Silkies for several years and currently working with bantam Ameraucana right now, had bantam Welsummer and bantam Polish before.. never YET seen a full size egg from any of them.
That being said... I do believe it's mostly a wash between bantam and large fowl... we use twice as many bantam eggs for eating but takes half the space and feed. I think the little eggs are fun... and half my family doesn't mind while the other half says it's more egg cracking and more figuring for recipes.
Between Dom and Rock... Rock eggs are bigger... and one of my favorite birds. Egg size in addition to how often is a good consideration... some birds might lay 4 smaller eggs per week verses another that lays 3 larger eggs per week. So it may not be just the size or just the frequency that matter... but a combination of the two. Pekin (Cochin) eggs were smallish comparatively.
If white isn't a good color... which I understand and agree, despite never losing a white bird YET (and that was what made her look so amazing with her huge red comb and little white body)... there are other colors of Leghorn. The brown Leghorn is a beautiful bird though not as prolific in production as the white.
Black Austra.. they look perfect on paper, no production experience as I only raised and sold one juvenile... but I also NEVER get egg numbers claimed by hatcheries... while free ranging. It's not true that happier birds lay more eggs.. Though it could still be true that stressed birds lay less... how does that explain the production seen in our egg production facilities. Genetics, coupled with lighting... water intake can increase or reduce egg size SOME.
Relative to picking a chicken that has the right cold or heat hardiness for YOUR location.. adding your general location to your profile might help folks make their best suggestions possible at a glance.