Summary
This quote humorously suggests that as someone becomes more arrogant or full of themselves, they may actually have less substance or capability to support their inflated ego. It implies that a person's self-importance can create a mismatch between the size of their metaphorical "head" (their ego) and their ability to effectively handle the responsibilities or challenges that come with their position or role ("filling their shoes"). In other words, the larger their ego grows, the less competent or deserving they may actually be.