It sounds like they could shoot harder than everybody else. I just have a hard time with the 118 mph. But they were clocking pitchers accurately back then too. Mickey Mantle was known for hitting balls 500 feet and he was Hull's size (5-11, 200). He had incredible strength and bat speed. Not too many guys, if any, hit them that far today. Reggie Jackson also hit long homers - 1971 All star game est. at over 500 ft. He was 6 ft , 195. Maybe that is the ideal size for power?
It's not the ideal size, but there's something very important to remember. In joints within the body, there's an inverse relationship between flexibility and stability. Someone who has extremely stiff hips is unlikely to suffer an injury to that area without application of extreme amounts of force, whereas someone with more flexibility is more likely to suffer an injury to that area (whether muscular, to the joint itself, or to the connective tissue).
Individual people also have unique physiological variances that simply cannot be fully known. I've known people with extra muscles that are far from standard and I've known people who have fewer muscles than normal. I know one who has an extra cervical vertebra, and one who has two additional thoracic vertebrae. I know someone who has a biceps tendon attachment on both sides that's quite clearly abnormal, and someone who has only two heads on each tricep. Heck, my father-in-law was measured just a few months ago to have a lung capacity of a very small amount less than Michael Phelps (and has consistently demonstrated the same results over a number of years), and his daily schedule consists of work and not much else.
Each one of these factors can have enormous impacts not just on daily life, but on athletic ability. A change in the attachment of the biceps tendons can have an incredible impact on exactly how much power can be derived from that muscle, either by being stronger or weaker. It is not outside the realm of possibility that both Hull brothers had some type of small genetic advantage that could have enabled them to fire a puck substantially harder than anyone else in history. When you consider that Brett was also known for an overpowering shot in spite of a physique that can best be described as "doughy", it would seem to lend credence to that idea.
As an aside, something else to consider with baseball is that hitters pre-1960 were using bats with much thicker handles and thinner barrels, ergo less bat speed, ergo less distance. With a modern bat, Mantle could have propelled the ball quite a bit farther.