Yahweh said to Gideon, The people who are with you are too many for me to give the Midianites into their hand, lest Israel vaunt themselves against me, saying, My own hand has saved me.
The character of ancient Israel being what it was, for any victory to have been achieved involving large numbers of them would surely have resulted exactly as the Lord here said to Gideon.  The great purpose of God in the defeat of the Midianites was that it might thwart the movement of God's people toward idolatry and turn them again to their allegiance to God.  For the accomplishment of that goal, it was absolutely necessary that the victory should be clearly that of God Himself and not by Israel in their own might.