Do I need to cut back on the plant growth in order to make my pumpkins grow larger?
No idea. Someone may be able to help.
Fertilise with something rich in potassium to help boost the size. Avoid fertilisers rich in nitrogen which will only promote foliage growth.
Don't forget to stop the plant as well you will have to pinch off the ends of any new stems and you can also help the plant retain energy by removing the tendrils.