Plus for every developer to come, one call to an external library usually also means 30 pages of documentation to trawl through if they ever want to change anyt

Plus for every developer to come, one call to an external library usually also means 30 pages of documentation to trawl through if they ever want to change anything, 29.9 of which is completely irrelevant to whatever your narrow use case is. That's the real cost. The size of the code means absolutely nothing.