Why does the Torah refer to the matters that Moshe was to deal with as "Davar ha'Kashah", whereas Yisro (in Pasuk 22) referred to them as "Davar ha'Gadol"?
Ma'ayanah shel Torah (citing R. Chayim Berlin): Yisro thought that cases that constituted small amounts of money would be brought before the judges, whilst Moshe himself would deal with big cases. Yisro did not know that, in Torah law, there is no such difference. 1 Therefore Moshe changed the distinction to easy matters and difficult ones.
Refer to 18:22:1:1 & 2.
Since, as the Gemara says in Sanhedrin, 8a, 'Beis-Din must give the same attention to a Din-Torah of a P'rutah as to that of a hundred Manah.'