Some of that is based on the size of the engine involved. To get much power from a small engine, you tend to have to run it faster, which tends to be less efficient. WHen you can run it through a transmission to keep the rpms at closer to optimum, you can improve the efficiency. SO, it's a tradeoff...larger engine, everything must be larger: radiator, muffler, springs, shocks, etc. This all adds weight, which affects the overall EV efficiency, especially in stop and go traffic. Keep in mind, the i3 was not designed as a highway cruiser...it was designed as a commuting, primarily city car. There, weight becomes critical. Once a vehicle is at its cruising speed, the drag is the primary load, but accelerating is where the weight makes a huge difference. F=m*v^2, so getting to speed, especially higher, cruising speeds, is a big part of the energy required. THis is why BMW stressed about keeping the weight down. A comparable Leaf (if you can consider one comparable), weighs about 20% more, and that's without an engine, compared to a REx, and significantly more when talking about a BEV. While people will argue about it, from what I've seen, the REx was forced on the designers by the marketeering group. At the beginning, and still, the biggest hurdle to adoption of an EV is range anxiety...throwing a more familiar engine in the mix with a gas tank, allowed it to get some more sales traction. Note, it appears that at least in some markets for 2019, with the newest, larger battery, they've dropped the REx as an option. The newest battery pack has about the same range as the original REx, but strictly a BEV. At least where your electricity is produced from non-coal plants, a BEV is greener than the hybrid version.
BMW has other, all EV vehicles coming out. The battery tech is now allowing decent size and capacity units at an affordable (relatively anyways) prices.
When it comes out, I may trade my ICE and i3 BEV in for an X5 hybrid. Maybe the end of the year, but maybe delayed until next year. ENough EV range for 95% of my needs, and long range for any trip I might want with good endurance. The other thing is that it could easily tow a trailer I'm toying with buying. The EV should really help with the starting torque, and the gas tank with the range.
The iNext and electric X3 should both have good EV range. There's more where they come from in our future...pretty much every car company is starting to get on the bandwagon...in some markets, they'll have little choice if they want to continue to sell new vehicles. Some analysts have predicted that last year was the peak of ICE production.
