Any idea on what breed my Chihuahua is mixed with?

Adopted him a few months ago and I got a lot of people asking if he's part German shepherd. Idk too much about dog breeds but would that be plausible? When my wife told me about him, I pictured a standard tiny Chihuahua, until I saw him in person. Obviously he's not HUGE but every other Chihuahua I've seen are tiny