What does "sweet spot" mean? [closed]

It's the area at which different factors balance out to give an optimal result. In this sentence, organisations below 300 do not benefit as much as those above that number, while also organisations above 1000 doe not benefit as much as those below it; so 300–1000 is the "sweet spot".

More formally, we might say that it was the optimum range.

The origin is probably from sports, particularly racket or bat sports where there's a "sweet spot" that the ball should ideally be hit with, then expanding into other sporting uses, before becoming a more general phrase.

According to the context of the sentence you presented, "sweet spot" is related to the point or situation where the best or optimal results can be achieved.

Pretty much the same as the sport meaning - the optimal size of organisation for Windows was ...