heavy

 light

 The elephant is the heaviest animal in the forest.

 You look lighter this year.

 weak

 strong

 Lisa is weaker this year. She should exercise more.

 I exercise a lot in order to be stronger next year.

 fast

 slow

 Betty is the fastest runner in the race .

 The tortoise was slow in the race. But he got to the finish line earlier than the hare.

 tall

 short

 John is the tallest boy in the class. He wants to be a basketball player when he grow up.

 Mary was the shortest child in the class two years ago. But she is the tallest now.