Pistons and rings expand when heated.... so the fit in the bore should be tighter when the engine is at working temperature. Pistons expand different amounts in different directions due to the influence of the structure through which the piston pin goes. Some are manufactured in Oval shape to compensate for this. I would expect compression to be less for the non turbo engine. You mention 'new' .... compression should get better up to a point after new... if proper 'breaking in' happens... then there should be very little change for hundreds of thousands of miles on a properly lubed diesel engine.