Actually, diesel engines don't necessarily have to have 'heavy' blocks. They need to have 'strong' blocks because of the high compression and vibration.

Now historically this has been one and the same, with only heavy grade cast iron being used for diesel engine blocks, while most upscale gasoliine engines have moved on to aluminum.

But Subaru broke the mold with the Boxer Diesel. Since the engine block on a boxer is just that - a square block of metal - they found that even aluminum in this configuration had enough rigidity and low vibration.

So the most modern diesels aren't any more complex, noisy, or heavy than a gasoline engine.