Just because an engine is bigger, does not mean it has more power, or can go faster.
In this case, I believe the 8.0 has more torque than the other engines listed. Basically, torque gets you truck moving, and hp makes you truck go faster.
Look at dyno results for any engine, nitro, gas, or diesel. Max torque comes at lower rpm's, where as max hp comes at higher rpm's.
This does not take gearing into play. That is a whole other can to open.