Thanks Gail,
I just noticed that instance1 is using 364 MB out of 6GB (it was 2.9 GB 6 hrs back) and instance2 is using 3.4 GB out of 6GB.(it was 763 MB 6 hrs back)
In which scenarios, sql server instance will release it's memory other than it's service restarts?
Min memory is set to zero. Do we need to set Min memory if we have multiple instances?