Which is a likely reason why a minor pharaoh like Tutankhamen was buried in such a magnificent tomb?

Becuase he was pharoh and king of Egypt

To compensate for his age and size ...

jk

it was to show how mighty of a pharaoh he was and how powerful and great he was. It was also so that he would more likely to make it into the after life and he could keep all his belongings with him