I don't think we have any proof of that yet, though? The story so far has made it seem like the appearance and name is just a very unlikely coincidence.
i suspect fake miyu is the big girl.
and now for a crack fueled theory:
1. fake miyu was in love with real miyu
2. on that day fake miyu confessed, but as you can see here shes very intense
3. real miyu killed herself
4. thats why the fat girl (fake miyu) is also on the list of suspects
5. she wants our boys help to catch herself, because she wants to be punished for being the reason she died