I'm a little confused by your question: space is not memory; space is the space being either used or available on your hard drive or SSD or disk. That is where the entire system, OS, and associated files are located. That also includes your apps and all of your files, movies, videos, photos, etc. Every time you save something to your Mac HD, it will take up more space - it's like a CD you add a song to.
Memory is commonly referred to as RAM and that is what is used to facilitate opening apps, working in them, browsing the internet, etc. You may have 8, 16, or more GB installed. There is no way that is 77 GB.
So, if you will highlight your Mac HD (that is your hard drive) and hit the Command + i keys, you will get a get info window with the space used and available listed. Please post a screenshot of that.
Also confirm your exact model/year of your device along with the OS version.