Science before the mid-20th century was too small to look like a target.

In this case I interpreted it to mean that there are many people now who specifically target science as bad (i.e. complain about "scientism" etc.) because science is a large, successful enterprise. He is asserting that before the mid 20th century science was not prominent or successful enough to bother being a target of attack. I'm not sure he's correct here, but the basic notion is plausible.