This has been my experience too. The 70g Spinworthy Steadfast does not strike me as a slow or heavy playing yoyo compared to some 65-68g yoyos, just for one example.
Yoyo qualities may be defined by basic physical principles, but they combine them in complex ways. If someone could make a mathematical model of a given yoyo, adjusting its weight or dimensions would have predictable results. Comparing two different yoyos based weight, size, etc. is a different story.
I think that defining/understanding these terms is still very useful because it can help define why a specific yoyo has certain qualities.