They have re-jiggered the meals lately, and I haven't flown in over a month (eek!), but IME there are small differences between domestic C and F meals.

On the DEN-IAD route, for example, I have experienced both C (widebody) and F (narrowbody) service. At dinnertime in C, there is typically a choice between one cold entree (e.g. protein salad, deli plate) and one hot entree (e.g. mahi-mahi, crusted chicken breast). In F, there is typically a choice between two hot entrees during the dinner time (e.g. tortellini, chicken breast).

Hope that helps!