Why does a tree produce more secondary xylem than secondary phloem?

In woody plants, the vascular cambium is responsible for producing both secondary xylem and secondary phloem through tangential cell divisions. The reason a tree produces significantly more secondary xylem than secondary phloem is that the cambium generates more cells internally (toward the center of the stem) than it does externally.1

This unequal production is a fundamental aspect of how woody stems grow in girth. As the vascular cambium continuously adds new layers of secondary xylem to the inside, it also increases its own circumference to accommodate the expanding diameter of the stem. Conversely, because the secondary phloem is produced externally, the older, outermost layers of phloem are eventually crushed, die, and are sloughed off as part of the bark due to the pressure exerted by the rapid growth of the internal secondary tissues. Consequently, the secondary xylem accumulates to form the bulk of the tree's woody structure, providing essential physical support and water conduction, while the secondary phloem remains a relatively thin layer.12