You haven't said how, as a middle power, they have influence on world events.
They have influence on other countries where they choose to have it, for their own reasons. The Japanese have never shown much interest in world events which don't affect them.
I do agree that under Harper, Canada definitely lost influence as a middle power.
Agree with whom? Yourself? Canada has no more or less influence in the world under Harper than under Chretien. Which is to say we have had very little for the last twenty years.
He was atrocious at international diplomacy.
No, he just didn't make it a personal popularity contest. If a lot of Muslim countries didn't like that he supported Israel, he didn't care.
Maybe he just had more self confidence than Trudeau, who seems to constantly require reassurance that people like him.