Google Chrome uses 3.8 GB of memory with 1 tab opened
If you go to Window->Task Manager in Chrome, it will pull up an activity monitor just for Chrome. Then, you can see if any extensions or sites you installed and are active are causing memory usage. 

You can also go to chrome://memory-redirect/ (or click the "Stats for Nerds" link in the Task Manager) to see this information partitioned in the following way (quoted from the page) for each extension/tab/process of Chrome:

"                  Resident:
                  Amount of memory that is present in physical RAM.
                  This is the best indicator of browser memory resource usage.
                
                
                  Shared:
                  Amount of memory that is present in physical RAM and can
                  be shared with another process.
                
                
                  Private:
                  Amount of memory that is present in physical RAM and can not
                  be shared with another process.
                
                
                  Virtual:
                  Amount of address space allocated in virtual memory."