What is the irony of the poem small pain in my chest?

The expression "small pain" is repeatedly used in the poem only to make sure we get the irony and get that well. As we go through the poem, it gets clearer that the pain was not at all a small one, rather it was a fatal injury that the soldier boy succumbed to. So the poet might be hinting that the injury of the soldier boy is not a great injury compared to the destruction war can cause.