You are confusing hard drive storage space with operational RAM memory.
So we can get accurate info about your iMac, go up to the upper left of the main OS X menu bar, click the Apple Logo, a drop down menu appears. Click " About this Mac"
A Pop up window appears with info about your iMac.
Gives us this info.
Also, in this window is a more info button, click this and a new window opens with additonal info about your iMac.
Cut and paste this info into a new reply editing out your name and the serial number of your iMac.
Also, to determine how much hard drive space is left on your iMac's hard drive, locate your main hard drive icon.
It might have your name on it or it might say Macintosh HD.
Click once on the icon. Then use the keyboard and hold down the command/Apple key next to the space bar and type the letter I. An info box appears that tells you how much space is used up on the hard drive and how much space is still available.
Do you sync any Apple iDevices to this iMac?