If you're only drinking one can then RedBull will have half the effect of Monster and Relentless because the can is half the size by volume. But if you drink the same about, i.e. 2 cans of RB vs 1 can of the bigger energy drinks, the effect will be the same.
What's more effective ... 180mg of caffeine, 180mg of caffeine or 180mg of caffeine? Exactly.