The easiest one to point to is the Mona Lisa. She doesn’t look like a real person to me, looks like it’s a result of stylistic choice and not supposed to be photo realistic. You can sit in Central Park for a week straight and nobody would look like the Mona Lisa. You see the puffy face and odd proportions a lot in old paintings.
Dutch Golden Age art is usually way better about this, and they tend to look like real, normal proportioned people. But then you can look at one like “Lady playing a viola” by Joost Van Geel and there’s NO way that lady looked exactly like that. Look at Johannes Vermeer work, people claim he used early camera technology to make the paintings, and Girl With a Pearl Earring looks pretty real, while Study Of a Young Woman does not look like a real human being... And I’m supposed to believe a camera helped make that.
What I’m saying is I know back then there was a lot of products of incest and maybe the gene pool wasn’t as strong, or maybe the painters just didn’t do justice to what people actually looked like, what is it?
Lack of knowledge, lack of proper tools and paints for the portraits, fashion of the time, and, more importantly, style choice. See this painting here? That's The Baptism of Christ, painted circa 1475 by Verrocchio. See the small angel further left and how it's so different from the rest of the figures? That's because that particular angel was not painted by Verrocchio (he had better things to do than occupy himself with lesser details. The idea of a lonely artist in a small attic, suffering for their art is very much a Romantic, 19th century ideal. Previously, they were crafstmen working in a workshop, with several helpers). That angel was painted by one of his interns, a very young da Vinci. Sure, his technic was still lacking, but his style, his taste was already there: long, flow-y locks, rosy cheeks, puffy faces, bedroom eyes. Very easy to detect.
You don't have to go back to the Renaissance to find the same situation: just get three superman comics from different eras and see how different Superman looks in each one, depending on the style of the artist, the style of the era, the artist's experience, the type of printing used, the price of each color at the time, and so on.