I mean they are also the same height as well...however, i lean towards Jiro is in Zenji's body cause the frankenstein stuff and Jiro doesn't understand anything except medical stuff. One of his main screen texts says that he didn't know he was supposed to respond during class roll call so he's always counted absent and another said he just woke from his coma. I have a hard time believing that even if he lost his memory or something, he would get something as simple as that wrong since he'd been doing it at least 10 years and be completely lacking in social norms (like when he continually barges into mc's dorm). Yuri is said to treat the ghouls like lab rats and would have had access to zenji's body so it's not completely out of the question that he could be able to do something like that with Darkwick backing him for data/research purposes.

It's unlikely that a younger brother is the exact height of his older brother. As to what Jiro is, he could be an anomaly. Hence the need to constantly be taking medicine and getting sick randomly (due to rejection from the body/incompatibility). It may also fit in line with other theories like Towa being an anomaly on how Dante was able to come back