Question: why are xylem cells bigger than phloem?
Answer: The required transport capacity of the xylem exceeds that of phloem, as the transport rate of fluids is higher in xylem than in phloem (Hölttä et al ., 2009). Accordingly, in most tree species, more xylem is produced annually. The ratio of phloem to xylem production varies between 1 : 4 and 1 : 10 (Fromm, 2013).