people don't train to get more chakra, it is limited of how much you get the moment your born, people train there jutsus so it takes less chakra and him spamming kamui is like naruto spammed rasengan and learnt it in 3 days, he got used to it.
Kakashi clearly gets better with Kamui over time. It took him ages just to use it once when they fought Deidara back in the day. Does it make real sense time wise? No but its just fantasy; the main thing is every time you see a main character fight, he'll be stronger than last time you saw him. DBZ style; you just get beat up and you're automatically tougher.