While these factors may contribute somewhat to the ultimate shape of the mature palms, I'd like to point to a more fundamental factor: the phase of development that all palms go through after germination called establishment growth. This is something peculiar to tree-like monocots, which have neither a taproot system nor layered secondary growth. In dicotyledonous trees, stem thickness increases gradually throughout the plant, and the root system branches to keep up with it. (See The Root of...