Lost coffin?

I just got done watching the Lost season 3 finale. I know that the coffin has Locke in it, but as I watch this, I don't see how. Isn't the coffin a little small? It looks like it's for a child .. when Jack is standing next to it I can very well tell that it should be larger. I haven't seen a coffin in real life, so I was wondering if anyone else thought the same as me.