Hi there,
Simply put, piston slap occurs when the piston is forced against the side of the cylinder wall due to piston to bore clearances. It follows then the more clearance there is the louder the knock will be.
Piston slap usually occurs when the engine is first started from cold. The piston, being made from aluminium has a different thermal expansion rate than the cylinder bore which is cast iron. As the engine warms up the piston expands at a greater amount than the cylinder bore, thus reducing clearance between piston and bore. As the engine warms up this noise should go away!

Hope this helps

CheeRS, Mike.