Crystals were mined from mountains and hills, and there were lots of different sizes to it. He got this big one, and he got much bigger ones.
Yet there wasn't a difference between the various sized crystals. They all had the same function, and the only difference would be in the total amount of spirit power stored within them.
William didn't know what caused the crystals he took before to explode. So, he decided to take out a much bigger one and see if it'd endure the spirit power surge from Angelica.
His best explanation would be in the high grade of her spirit. After all, his test would assess one's spirit and spirit power. Even if she was weak right now, her spirit was still the same as the one she had as a monster.
