Taking out the 15% buffer, that means the i3 gets 114 miles on 28.5kwh of charge. That means, without adding weight (2961 lbs), the i3 can go 228 miles on 57kwh! The bolt would have a 5% buffer in that case!
Either the 60kwh is usable (66kwh raw?) or the bolt is significantly more aerodynamic than the i3.
My bet is on a significantly larger battery.
It's not just a bigger battery. The MPGe ratings are quite high, so the GM claimed efficiency is higher than the much lighter i3.