Could be a few reasons. Mislabeled plants from the nursery or other seed mixed in your packet of seeds . (This happens often nowadays I find.) Or it could be the weather. Where DO you live? Important to know that.

My tomatoes are smaller and later this year due to colder summer temperatures.

N. California (z9). They were normal size for about 2 months--but now much smaller, somewhat oval and not as tasty as they were initially.