Let’s say the price of bitcoin is $10000. You want to go long using futures, so you start buying contracts. A few people will want to sell futures at $10000, but not for a trillion dollars. If you want to buy more futures, you will have to pay more. If you offer $11000 there will be more people willing to take the other side of the bet. And if the price of the underlying is still $10000 arbitrage is possible. I could buy one bitcoin for $10000 and sell you the future for $11000, pocketing $1000 no matter what the future price of bitcoin may be. That creates demand for bitcoin if there is huge demand for futures, pushing prices up.
