A very powerful image showing a batallion before and after the Great War, the upper

A very powerful image showing a batallion before and after the Great War, the upper picture showing the size of the group after they enlisted, the lower picture after the armistice was signed
(TITLE FROM REDDIT)

Let's put it into perspective. The U.S. has suffered 57,000 casualties in the GWOT.
The British lost that many in the Somme. On day one.

Were they firing people out a canon?

Actually the Battalion would have received reinforcements throughout the War, though as the war went on it became increasingly difficult to keep units up to full strength. This is probably a picture of who was left of the "Old Originals" of the Battalion, meaning who was left of the original 1914 group.