First of all the seed you use is very different (basically big seeds
grow big pumpkins and small seeds result in small pumpkins). Also, large pumpkins need more room to grow, 40 Sq. feet or more and small pumpkins can get along with 20 to 25 Sq. Feet. Lastly, very large pumpkins don't like moisture stress (too little rain).

Absolutely, you need bees for pollination of pumpkin flowers. In
fact, to get uniform shaped pumpkin fruit the pumpkin flower must be visited many, many times (about 30).

For me, keeping white-tail deer out of my field is my greatest
challenge. Next is controlling disease. I use a fence to control deer and it work well. Some other farmers a dog or dogs to patrol their field. They keep the dogs in the field with an electronic collar and an invisible fence. To control disease we use special cultivative practices and spray with preservatives.

It is not a given that large pumpkins have more seed - they do however have bigger seed typically.

I was told by a school class this year that counted the seeds in small and medium-sized pumpkins that the small pumpkins they looked at had considerably more seeds than the medium-sized pumpkins.