Cyril Connolly (1903–1974), British critic. The Unquiet Grave, pt. 2 (1944, rev. 1951).
There is a similar observation in George Orwell's Coming Up For Air, pt. 1, ch. 3 (1939): "I'm fat, but I'm thin inside. Has it ever struck you that there's a thin man inside every fat man, just as they say there's a statue inside every block of stone?" For a later variant, see Amis on obesity.