The gemstone born within every monster absorbs energy from outside resources to strengthen the being's power. Not every monster can absorb the same amount of power. Some can take more at once, some take less. As well as the strength of every monsters maximum form won't be the same. Pretty obvious, a monster reaches maximum form when they succeed in taking in all the energy their body can handle. Pretty simple. Anyways woah actual art, yummy yum I want pizza.