Have you looked at the bottoms of the fruits? If not do so. Romas get blossom end rot (BER) quite readily, a condition that triggers early reddening.

If that's what they have, check the FAQ at the top of this page, then scroll down to BER.

Thanks for the tip. No end rot. They may be small but they are perfectly formed.

Full size is not exactly a fixed size. For me for example, full size is about 2-3 times smaller than average size advertized. Growing conditions will greatly affect the size.
If you mean smaller than what you had other years, it can be cauzed by less fertilizer or improper fertilizer, less leaves (due to some fungus for example), and probably some other factors, for example not much sun.

Plant stress will also result in ripening. Have you ever noticed that an unhealthy plant will produce, if it produces anything, small, early fruit?