Well Onix, there are several reasons for that.
On one hand, after having been chemically castrated by his wife, Jacob does not feel very secure about his manliness. So he likes to use big phallic symbols.
On the other hand, despite his bulk, he does not really have a lot of muscle and the wooden hammer is much lighter than a big ass iron one.
And on the third hand, the big hammer made it easier to hide the fact that he is not exactly hitting those nails heads in these pictures for me.