The Sunbeam Lotus springs to mind, which hides a twin cam 2.2 litre engine in a three door hatchback shell.  The Rover R8 had a twin cam 2.0 litre engine, again in a smallish hatchback body. My father in law once had a mid 2000s shape Clio fitted with a 1.9dCi engine, admittedly in a 5 door shell. What’s the biggest engine in a small car that you can think of? Modified cars don’t count.