Hello! We actually discussed about this in an art history class some time ago, and I will answer your questions to the best of my memory. So, for the ealy and imperial era, they were in search of realism, and in the late roman portraiture, they were in search of symbolism.

The art canons in between the wealthy and people were not the same, and the portraiture shifted to bridge that gap and say that the emperors knew the culture of the people. Also, it was more cartoonish to reflect symbolism, as said earlier. For instance, the bust of flavius Arcadius had large eyes and large ears, he sees and hears his people. How do we know it's a shift in style? It's because we can see a clear line in between this and medieval art, which is to be understood mainly by symbols, not realism.

English is not my first language, and art history not my specialty. If someone knows more about it than me, don't hesitate to comment