It is a big change in imperial currency. And it must have been intentional.
Centralized minting in Rome at the end of the julio-claudians may have caused loss of skill in the western provinces. But we know that the old bronze was still in circulation. There are many treasures with mixed coins. The models were there to be copied if they wanted.
Was it a change towards very low relief for technical reasons? Because the coins were so much thinner that the old style portraits would always come out weakly struck?
Perhaps. I noticed that many of these nummus appear weakly struck.
That would explain why in this one the reverse figure being smaller conserves more of the old style. It needs not be very high above the field to have a fuller style. The big portrait was the feature that got flattened in these thinner coins.