Yeah, these trucks have an larger cab.
Compare an HMMWV with this ECV Pictures in the original post, note the height of the C Pillar vs the pickup bed. Much more headroom, Better armored on the bottom.
Hood is kinda funny looking but it's a prototype, early HMMWV hoods were goofy looking back in 80-82. It's due to a new cooling stack/radiator layout as someone mentioned.
Frames upgraded and is now made up of 3 pieces bolted together into one rail as opposed to one rail. I guess it's easier to swap out frame sections if the front end is damaged.
Engine is a tuned up 6.5 Optimizer, better cooling, differentials are now actively cooled as well, not sure if they are locking or not. GVW is about 18,000 lbs or so. The more recent trucks, H1's and A2 HMMWV's had a GVW of 10,300 or 12,100 lbs, but by the time you filled it up with armor and equipment, you have already used up your 4000-5000 lb of cargo carrying capability.
Bumping up the GVW would allow an uparmored truck to still carry a few thousand pounds.
The 2 piece rims are upgraded to handle the extra weight.
I've seen some bits from the ECV II hit civilian hands for sale.
One guy on the HML had the 3 piece sectional frame and underpinnings for sale from a project, and I've seen those rims pop up on government liquidations I believe.
Even with FTTS and the other new trucks, the HMMWV in one form or another will be in the fleet for decades to come.
There is also the HMMWV repower program, AMG and the Military are looking for something to replace the 6.5 down the road.