comparing minetest (a c++ minecraft clone) and minecraft gives the following memory usage upon starting on a blank new world. Also, minecraft takes five times more time to launch.

  smem  | grep mine 
   5728 jcelerier minetest                           0   433136   439287   466348 
   7044 jcelerier java -jar /usr/share/minecr        0   691476   708501   753948