It depends on the size, but a vintage A-2 or G-1 generally weighs in the 3-4 pound range.
Vintage jacket leather was more likely to be 2.2oz, not the 3oz that is often popular today. I personally prefer jackets that weigh under 4 pounds or 1.7 kilos.
Weight indicates the jacket's been made from a thick(er) leather, which of course makes it tougher and more durable I'd imagine, but as far as the quality of the leather itself goes, I don't think weight has got much to do with it.
I hear you but as I say 2.5 oz is more than thick enough to wear like iron for a lifetime and never tear like that piece of **** Belstaff.
....... I suppose there are fashion versions of Belstaff leather out there but I haven't seen one in the wild.