Interesting.
From a teaching relative I have heard that at school level science subjects are simplified to the point of basically being wrong. This would suggest at degree level the sciences step up significantly.
I also heard from the same relative that whilst at University as a scientist she noticed that students of arts degrees appeared to have a smaller workload and fewer lectures. But, she's considerably biased. And I don't think this applies to subject such as Geography which bridge the gap of science and art.
However, numerous science-minded peers at school have described my subject set which includes Higher English (English Lit. for those in the English system), and two Higher social subjects as "hell" due to the workload, amount of revision, the amount of writing and the harsh time limits of the exams. But then these people may be lazy and find themselves screwed when it comes to University.
A science-minded friend of mine once said to me: "to be good at Maths you just have to be smart. To be good at English you have to be smart and deep." I think there is some truth in that. Arts subjects require imagination, creativity, analytic thinking, an ability to think outside the box and, in some, a certain poetic philosophical-ness. Some (but not all) STEM students have this.
Therefore, I think at the end of the day science subjects have a higher workload at university level than most arts subjects. But, for arts subjects you need a set of thinking skills that some science students just don't have. Thus, both are the same difficulty.
(Sorry for the massive post)