Whenever you left and join a world, while still having minecraft open, minecraft will start using more Ram than previously when you had your previous world opened (including the same world you played on before even if no changes have been made)

Whenever you left a world and then join to any world you have in the menu, minecraft will use more RAM then did previously when you had your world opened, basically, the more times you leave and join worlds while still having minecraft open, the more RAM minecraft will use. This happens even if nothing changed in your world.