they're considered "commercially seedless" mine had some seeds this year.

size can be a combination of different factors like fertilzer, irrigation, heat, overburdened with fruits, etc.

Some citrus have alternating years of production. Don't know if this is applicable here.

reneezies, if you had heavy fruit set, then the fruits will be indidivually smaller. And, navels sometimes have seeds, that's not unusual. As long as the fruit is still sweet and good to eat, I wouldn't worry about it. Just make sure you fertilize properly this season, and if you have exceptionally heavy fruit set, thin the fruits at the BB size or marble size to encourage larger fruits.

Patty S.