Using an example of the game Zoo Tycoon, the game will give you a notice that ''There is not enough food for the animals.'' Or ''There is too little food for the animals'', both meaning the same thing that in the animals cages the food supply is too low. If you were to say ''It's too little food for the animals'', you would usually be directly telling someone, maybe someone who is putting out food and hasn't put out the right quantity, or someone who has brought home a small amount of food. The difference between ''There is'' and ''It's too'', 'there is' being more circumstantial, while 'it is' is like saying ''That is too little food for the animals.''. I don't know if that makes sense but maybe it will help.