how does that square with this saying: "penny wise, pound foolish"?
Often a proverb will have an opposite proverb (though the example you give isn't a proverb). For example “too many chefs spoil the broth” and “many hands make light work”
The ideal amount of help then clearly rests somewhere between "too many" and "many" :)
Though maybe the proverbs are not exactly opposites. One is about the quality of the outcome, the other is the speed in getting there. Perhaps that's the lesson.
sancho panza in don quixote took this to a ludicrous extreme. they're known today as "sanchismos".
"Pound foolish" is not a necessary outcome of "penny wise", in this saying, but "the pounds will take care of themselves" is. Taking care of the pennies works when one is, at worst, "pound neutral". But being "pound foolish" is an additional condition in the saying, which is not accounted for in the "take care of the pennies" saying.
Project management version: look after the molehills and the mountains take of themselves