It was inevitable, given the conceptual and empirical content of development economics in the mid 1970s, that the early World Development Reports painted with a broad brush.
paint with a broad brush is an idiomatic expression. The sentence simply says that the early World Development Reports regarded things in general terms without getting deeply into the specifics.
Look here for the details (it has got some very good examples under the quotations section). Here's specifically the part of interest:
paint with a broad brush
(idiomatic) To describe a class of objects or a kind of phenomenon in general terms, without specific details and without attention to individual variations.
The idiom having been explained, here's the picture of a broad paintbrush for, as TRomano mentioned, painting crude lines:
To see why it is broad, here's the picture of a fine (thin in thickness) one for painting small details: