EJH, not quoting all those pics again but you've compared opposite sides of the Life Centre! The front actually does look similar to the CGIs:
All I know re the Grandstand is that the architectural drawings show the new stand to be substantially taller than the horseshoe. At a guess I'd say the increased legroom, splitting of tiers, corner buildings cutting out about 2k of potential capacity, and the truss and roof adding to the height, are factors.
Regarding the legroom, at Southend on Saturday it felt like I was in a 5-6k stadium. The stadium actually holds 10k, but the legroom had to be a huge factor in that. I was 'sitting' in a space that simply wasn't big enough for one grown adult.
The point is, legroom is going to be bigger in the new stand and is definitely a factor in the stand looking bigger than other c5k structures. Now for those padded seats....