I would argue it is much harder to maintain a good GPA in my major (physics), than it would be in English. In an English major, it may be tough to get a 90 in a class, but I would argue it is relatively easy to get an 80 or 85. In physics, I would argue it takes much more work to get an 80 in a class, and still takes more work to get a 90 or higher than it does in English. As evidence of this, my last four upper-level physics classes had class averages of 59%, 61%, 39%, and 50%.
I would, however, concede that once you have already reached 90, it would be much harder to change that mark into a 97 in English than in physics. (However, at my school, there is no difference between 90 and 97 to a GPA--both are 4.33.)
This likely varies from school-to-school however. Math, at my institution, is much easier than physics--class averages tend to hover around 75% rather than 55%. It would be quite easy to maintain a high GPA in math relative to physics at my school.