SharePoint's SQL Server using High Memory (RAM)
Please refer the below links:
SQL Server is designed to use all the memory on the server by default. The reason for this is that SQL Server cache the data in the database in RAM so that it can access the data faster than it could if it needed to read the data from the disk every time a user needed it. If we needed to go to the disks every time that SQL Server needed the same data over and over the disk IO requirements would be insanely high.
So if you want to fix the memory, then this step can be followed: