Answer:
Because of phototropism.
Step-by-step explanation:
Phototropsim can be described as the action / movements of any plant, while growing or later, towards or according to the position of the sun. The oak tree that was planted in the forest needed to grow taller to be able to get the sunlight it needs among other trees so in the end it is taller and therefore thinner. Whereas the oak tree that was planted in the open field had direct access to the sunlight it needed and it grew as shorter and therefore thicker.
I hope this answer helps.