I see from checking out cars e.g.

the face lifted Octavia petrol

the face lifted VRS

and comparing their weight stats

Are the same model Diesel cars by default heavier than petrol cars ?

if so, why ?

Yes, not unusual. Diesel engines tend to weigh more.

More extensive use of heavier metals used in their construction typically.

so diesels are better built : ) )

Not sure about better, just heavier....

If heavier equals better in your book then

They're heavier because they carry a tankful around for longer.

Runs and hides....