This question relates to an early part of the book, when the family is first exploring the island. After their crew abandoned them during a storm, the entire Robinson family was spared with a number of animals and provisions intact and were forced to relocate to a tropical desert island. In contrast to many subsequent stories operating on the same trope, the Robinson family is comparatively upbeat, optimistic, and even curious, identifying birds as they begin to get their initial bearings around the landscape of the island. In contrast to our modern exposure to the "island stranding" literary trope, it almost seems humorous.
The family then begins to approach a wooded area to explore. However, they are immediately "taken aback" when they reach these woods due a complete subversion of their expectation. What they had thought was a relatively large woodland area was in reality only a group of just over 10 trees. However, the trees were so outlandishly large that William Robinson had originally thought they signified a much denser forest. Robinson is completely enraptured by their beauty, saying that they seemed to be "growing in the air." He asks his son to climb the trunk to measure its width, and the task is said to have taken more than 40 yards of thread to achieve.