When you compare the upper stage of an Atlas 5/Delta 4, which use an RL-10 upper stage engine vs a Falcon 9 upper stage that uses a Merlin 1D Vac version, you see really big performance differences.
The RL-10 is a LOX/LH2 engine with great specific impulse ($I_\text{sp}$ of 430-450 s depending on version) but crappy thrust of only 20-25 Klbs.
The Merlin 1D vac is an LOX/RP1 engine with so-so $I_\text{sp}$ (good for RP-1 though) at 304 s, but much better thrust at 180 Klbs.
Yet, the RL-10 is considered a much better upper stage than a Merlin 1D.
Sure, that is a pretty big $I_\text{sp}$ gap (25% more for RL-10), but with so much more thrust (8-9 times more for Merlin), it seems like it should not be such a big deal.
I understand that $I_\text{sp}$ is a measure of efficiency, in terms of how many seconds of 'use' you get per unit of fuel. Thus an RL-10 is 25% more efficient than a Merlin 1D. But as long as you are carrying sufficient fuel/oxidizer and have the thrust, why would the 8-9 times different thrust values matter more?
Ultimately this is probably a question of why $I_\text{sp}$ matters so much in this context?