I'm running some tests on my old and trusted Raspberry Pi A+. 
I've installed Raspbian - latest release, lite version. At the prompt, it's using 92 MB of RAM, mainly due to systemd and NetworkManager. As soon as you use it (even just for apt), it starts swapping and becomes almost unusable. It took 5 minutes just to install Python. 
I tried NetBSD (on the same memory card): 35 MB of RAM used (including Postfix!) and it's totally usable.
 
      
  
             
      
  
             
      
  
                            
                        
                         
      
  
                            
                        
                         
      
  
             
      
  
                            
                        
                         
      
  
             
      
  
             
      
  
             
      
  
                            
                        
                         
      
  
                            
                        
                         
      
  
             
      
  
             
      
  
                            
                        
                         
   
      
  
             
      
  
               
      
  
             
      
  
             
      
  
            