Majority of humanities subjects are heavy in content - this should be no surprise.
I took both MH and Economics and because I enjoyed them so much, the content load didn't feel stressful at all as they were interesting. However, if I must pick one as the heavier subject in terms of the amount of information, it would be history but I guess this also depends on the teacher.
History is heavy in the sense that the questions can ask for a lot of things over a long period of time, esp the extended response. While economics had less content, they are without a doubt, more difficult than MH. The theory are much more complex to understand, than, say business whereas history is merely memorising and the ability to write a good essay.