For example in books or stained glass windows, people have big, pointy faces and thin limbs, whereas most Greek/Roman statues or reliefs look very life-like.
Especially when discussing something as subjective as artistic style, one should be careful of making such qualitative assessments. For example, Picasso is considered one of the great artists of the western canon, and as a modern artist, he certainly had access to the materials and techniques to represent the world more "realistically" than he did.
u/Guckfuchs has a great answer to this question here: https://www.reddit.com/r/AskHistorians/comments/74eedm/why_did_prerenaissance_christian_art_degrade_so/dny8p1m
/u/sunagainstgold and I answered a very similar question HERE. There is a great deal of discussion about the production of art throughout the Middle Ages and the aesthetic values held by the artists and viewers.
Others have linked threads that talk in a general sense about medieval art, and I wrote an old answer about how artistic convention manifested in a specific genre; in this case, the depiction of the physique of knights and fencers.