Coastal Banksia growing really high with out filling out to the sides

I have several Coastal Banksia trees (Banksia integrifolia) planted around 2 years ago. All of them have grown several meters straight up with very little side growth. Is this to be expected? I am very close top Port Philip Bay in Melbourne’s inner Western suburbs. My soil is very sandy. I only occasionally water the tree when we aren’t expecting rain for an extended period. I have not applied any fertilisers etc since planting. Other than that it fends for itself. Any ideas what has caused this type of growth and is there anything that can be done to have the tree fill out to the sides at all lower to the ground?