Response time - Mongostat command

Hey, at first the Memory amount for Mongod was 250 mb, the inserts per second was around 8000, when it was modified to 2 G, the inserts per second is almost the same, but it finished faster why is that?

and the dirty memory was %4, used was %86 before modifying, after modifying both dirty and used are around %4, why dirty didn’t use more than that?