For some reason I thought they were bigger. My Sonic actually looks slightly bigger
The numbers are actually pretty close. The interiors are similar, the Mazda is slightly wider. Overall it's longer, but the Sonic actually has more cargo volume and weighs about 200lbs less
Here's the Mazda's dimensions:
And here's the Sonic
Also, and I hate to say it, the new Mazda 3 hatch looks far worse in person than in pictures. The front is alright but the back is pretty ugly in my opinion. The interior looks nice, but overall I think this was a huge swing and miss by Mazda. The sedan is better looking.
All that said though, a new speed 3 with this small hatch would be pretty friggin amazing.