I would have assumed of course that the size limits were because the passwords were being stored in plaintext in fixed-length fields, but I guess they wanted to make sure they were 'complicated' enough? I guess salted md5 is literally better than nothing.

The character limits for usernames, though... smells like a SQL injection issue. Which is an obvious and completely naive thing to assert but they're using PHP so my immediate thought is that they're passing raw userdata into the database as strings.