Cudamalloc increase the commit memory in win10

when i allocated gpu memory by cudamalloc at program the start up. for example, 600MB, then I can use the memory without call API with small latency. But I find the commit memory size in task manager increases 600MB or more.
the pysical cpu memory has no increase.
could you tell me the reason? I use the win10 x64 and cuda 10.1.