When I change terrain size even by 01 x 01 it maxes out my PC!???

Hi, I'm somewhat new to Unity - .. I'm trying to create a world thought, and every time I try to change the size of a terrain - it ends up pushing all 4 of my pc cores to 100%. It does this if I change the width, length, or height at all. It doesn't matter if I only change it by 1 on each - it still starts maxing out my pc and just keeps maxing it out until it gets above 70 celcius - at which point I have to shut down the program. Anyone got any ideas how to fix this please??? Thanks.