Shree Hanuman

As a young monkey god, Hanuman was quite naughty. He was also quite unaware of his powers and stature. This made him use his powers to pester the saints living nearby. There are lots of Hanuman Photos available that show Lord Hanuman using his powers to  create whirlwind with his breath, pulling a supporting stick with his tail, pulling a beard and dousing a sacred fire. With passage of time, Hanuman became very naughty and all the saints and gods grew vary of his tantrums. In order to rid themselves of the naughty troubles of Lord Hanuman, they petitioned Lord Brahma to find a solution.  Thus a curse was created to protect the world from the mischief that young Hanuman created, by removing his knowledge of his powers.
 
This curse made Lord Hanuman forget about his powers and thus he did not trouble people anymore. The Power of Hanuman - of which becoming big is just one example - only became available again when Jambavant, King of the bears, reminded Hanuman that he has that power.