This is actually a common problem with digitisations of metal fonts, not specific to Courier. Many (although not all; Adobe's been pretty good about avoiding th

This is actually a common problem with digitisations of metal fonts, not specific to Courier. Many (although not all; Adobe's been pretty good about avoiding this particular failing, for example) of them are too anemic for screen use at body text size, presumably (although I have no hard evidence) because of not taking account of ink bleed. Victims include Bembo (the first one, not Bembo Book, which partially fixes this), several Caslons, Sabon, Janson, (an extreme example) Simoncini Garamond, and even Computer Modern (where the problem wasn't so much a transition from metal to digital, since Computer Modern was designed digitally, but from METAFONT - which can be parameterized, allowing "blackness" to be adjusted for ink spread - to Type 1 - which isn't). Speculating a bit, one of the problems that digitisers may have encountered is that ink bleed affects fonts differently at different sizes. At small point sizes, ink bleed is proportionally large relative to the stroke width; at large point sizes, ink bleed has negligible effect. This is actually exactly what you want for readability's sake: at small sizes, you want to avoid thin strokes, and ink bleed does exactly that. At large size, "thin" strokes aren't actually thin any more. But a perfectly scalable digital font does not capture this beneficial effect of ink spread.