why are the waves so big?

waves are basically the transfer of energy from one point to another with(mechanical wave) or without(electromagnetic wave) any material medium.in a wave disturbance created by a particle is trnsfered to another particle and this goes on till infinty.often particles oscillates and the energy is transfered from on poin to another.

That completely depends on the areas terrain features. A sudden steep at the beaches into the ocean may be one reason. And the gravitational pulls of sun, moon and earth is other reason.