No way is the Volt rear seat and cargo room worse than ANY sub-compact, including the i3. I like to think of it as more comparable to a Bently Contentinal Flying Spur with the four passenger back seat option.
I was shopping for subcompact hatches for a friend and saw the rear seat and cargo specs are significantly better for a lot of the popular subcompacts vs the Volt. This is esp. true after you factor in how the Volt's hump eliminates hip room (the only place where the Volt tends to have significantly higher measured numbers).
2015 Accent
Rear Seat Room in. (head/shoulder/hip/leg): 37.2 / 53.4 / 47.2 / 33.3
Cargo room (cu ft, cu ft seats down): 13.7, 47.5
2015 Fit
Rear Seat Room in. (head/shoulder/hip/leg): 37.5 / 52.6 / 45.1 / 39.3
Cargo room (cu ft, cu ft seats down): 16.6, 52.7
2015 Versa Note
Rear Seat Room in. (head/shoulder/hip/leg): 38.0 / 51.9 / 48.8 / 38.3
Cargo room (cu ft, cu ft seats down): 18.8, 38.3
2015 Yaris
Rear Seat Room in. (head/shoulder/hip/leg): 37.6 / 51.8 / 50.1 / 33.3
Cargo room (cu ft, cu ft seats down): 15.3, ~30 (Toyota doesn't publish)
2015 Volt
Rear Seat Room in. (head/shoulder/hip/leg): 36.0 / 53.9 / 51.2 / 34.1
Cargo room (cu ft, cu ft seats down): 10.6, 18