In joint training between two armed forces, there is usually one common goal between soldiers. While the official reason is to better cooperate in times of conflict and real operations, the off-handed reason is to flaunt their worth to each other. When you are in the armed forces, you are representing your country. Nobody wants to appear weaker than the armed forces of another country.
They decided, why not have a friendly tug of war battle? The Canadians wanted to get rid of their bad reputation so they couldn’t wait to destroy some Americans in Tug of War. All in good fun of course. The skinnier Canadian soldier picked the bigger American to show America what was up.