From G.K. Chesterton's Wikipedia page, there is the following anecdote:
Chesterton was a large man, standing 6 feet 4 inches (1.93 m) and weighing around 21 stone (130 kg; 290 lb). His girth gave rise to a famous anecdote. During World War I a lady in London asked why he was not 'out at the Front'; he replied, 'If you go round to the side, you will see that I am.'
What does "If you go round to the side, you will see that I am [at the front]" mean? I'm not getting the humor...