This is an oft-repeated theme in the novel. Porthos is described most frequently as a "giant," a "Titan," and a "Hercules." Being crushed to death under an immense load of rocks is therefore an apt mode of death for Porthos. As he manages for a time to delay the inexorable downward weight of rock, his figure is reminiscent of the immortal Atlas in Greek mythology, who bears the weight of the world on his shoulders.
King Louis XIV as France, the Sun, etc.
Kings are supposed to be the physical embodiment of their nation, and King Louis XIV uses this fact to his full advantage when arguing with his courtiers. On his way to being known as the "Sun King", Louis deliberately uses various metaphors for power to consolidate his rule as an absolute monarch. It is historically accurate that Louis XIV was known as the "Sun King," and that he sought to create an absolute monarchy.
White Horse, Black Horse
If you're so inclined, you might want to analyze the fact that D'Artagnan rides a black horse to chase Fouquet, who is mounted on a white horse. Might it signal Fouquet's innocence and purity and D'Artagnan's lack of these qualities? It seems like a bit of an obvious symbol to us, and we think Dumas may have selected those colors simply for their aesthetic qualities, but if you want to find some evidence for it signaling a good guy/bad guy dichotomy, have away at it.