It can't. There pihole only sees your DNS requests, not any other traffic, so it can't tell if you pulled 100 KB or 100 GB from that domain you just looked up.

It does show you DNS requests counts and such per device, but that's a poor proxy for bandwidth.

It's a DNS server so it has no way of knowing the bandwidth usage of the clients, the traffic is not flowing trough the pi-hole, it just responds to the DNS queries sent to it by the clients.