I define scaling as the ability to do something better than the enemy opponent in late game when not being focused down as a squishy or when being focused down as a tank for example.
Scaling is the net worth your character has and how much it can contribute to a fight potentionally via DMG.
When we talk about scaling we usually focus on Pure breed carry champions who have little to no utility.
There is no doubt a Kog Maw has more scaling than a Sivir in a 1 vs 1 scenario.
However Sivir has utility that can strengthen her other carries to do better, however this is not true scaling its just being an enabler like how Ahri`s mobility enables her to assissinate, but she will never reach the same numbers of damage as an equally good Cass, this is because Ahri has to sacrifice dmg for mobility and Cass has to sacrifice mobility for dmg.
What decides lategame more than anything is team composition. If your team can cover some weaknesses and have some members carry certain parts of the game up until lategame its all down to execution, throws or by simply having a better late scaling comp.