I think this question might be missing the point of TOTP and protection it provides. The reason 256/512 is used to encrypt data and passwords is to prevent the possibility of brute force and other attacks (e.g. using other data breaches). This doesn't really matter with TOTP. They can't reverse engineer a TOTP password out of you. They can't use your info from prior breaches to gleen what your TOTP might be anywhere else. It's not something where "cracking" the hash is likely to be attempted, as an attacker would still have to capture the generated codes and time of input in some way, then brute force hashes until they generate one that produces the correct codes at x time. Why would they ever do that when it would be a thousand times easier to compromise a device or TOTP app, and scrape the hashes directly from it; negating any need to brute force?

Note: I am not a cryptographer and have not implemented a TOTP server, so I could be completely wrong.

TL;DR 256/512 wouldn't necessarily increase the security of TOTP at all.