Perfection is being able to run at your limit of grip with the track.
You can go faster if you can raise the limit of grip and have the hp to meet your new limit what ever your weight.
In every case if your limit of grip lowers you cannot go as fast.
Beyond the above it's about if a heavier weight raises their limit of grip enough in the turns and picks up enough additional maintained momentum in the turns, to over come the acceleration advantage on the straights of the lighter racer.
The reality of it is about if the heavier racer is skilled enough to free up his heavier racer.
When you see the heavier racer build momentum and carry more speed thru the corners beating lighter racers, it's not about the heavier racers skill or ability to have more grip in the turns, it's about the lack of skill of the lighter racers.
The track offers the same grip to all racers.
In all cases a lighter racer with stickier tires should beat a heavier racer with less sticky tire because all other things being equal the lighter racer builds less g force and can run the same line faster. ... and a big maybe on that one. ...
To maintain the same speed with equal hp on the same line the heavier racer must use more grip then the lighter racer.
If there using more grip then they also have more rolling resistance.
Again it seems to me it's not about the heavier racer having more skill it's about the lighter racer not having enough skill.
I once watched a match race between the two fastest super modified's and the two fasted winged sprints on paved oval.
The winged sprints totally dominated the supers in both acceleration and in the turns.