"pagefilesys" is your windows paging file. It's basically virtual ram that sits on your hard drive ready to be used.
PAGEFILE.SYS is the virtual memory file Windows uses. Typically, on install, Windows sets the size of the file at around 1.5 times your physical memory size however this size will vary depending on the amount of free space on the disk when the file is established and other factors. Most will find the default size works fine but it can be changed.
You can set the initial and maximum size for the pagefile.sys in Windows through the System Properties. Right-click on "This PC" or "My Computer," select "Properties," then click on "Advanced system settings." In the System Properties window, under the "Advanced" tab, click on "Settings" in the Performance section, then go to the "Advanced" tab in the Performance Options window. Here, you can adjust the Virtual Memory settings to configure the initial and maximum size of the pagefile.sys.