Stop in for a cup of coffee

Ok guys, I need help wrapping my head around something and I didnt want to make a thread. So explain to me why when you use a carb sizing calculator it gives your what your ideal carb size is.. For example my engine is spec'ed to use a 625cfm carb. Ok now you have a factory 340 six pack and from my research is around 960cfm total.

I was running a 750 on my car and it ran good, but when I threw on a 600cfm carb it was much more responsive..Regardless of that, what I dont understand, how is it possible to oversize a carb?? Why could I not run a 800cfm carb on my car and have it run like a six pack? Is what Im saying making sense?