Meager means small and often applies to portions. Your mother may only allow you a meager serving of chocolate cake, and you may need to eat a meager serving of veggies in order to deserve that dessert.
Meager doesn't mean adequate — it means not enough. People with a limited supply of food may try to subsist on meager portions. People who make a meager amount of money probably have a hard time making rent. Meager and emaciated both derive from the Old French maigre. If you can remember that you will look emaciated if your diet is meager, you’ll use the word correctly.