Many science professors hold it as an article of faith that students do far less of the reading in their classes than they do in humanities and social science classes. I heard this expectation expressed at the APS workshop for new faculty I went to several years ago, and in other presentations I've heard about physics and astronomy education. The technique Just In Time Teaching was invented partly as a way of allowing science classes to make better use of textbook reading. Is it not a waste to spend classroom time in information transmission, telling students in a linear fashion what they could just have easily read from the textbook? Physics education research has shown that active learning is much more effective in getting the students to really understand the concepts.
When I've heard talks about this, the view I've heard expressed is that it would be crazy to expect students to come to a literature class without having done the reading. They would be completely unable to participate in that day's discussion. On the other hand, the view is, the norm is that students don't do the reading for their physical science classes, except perhaps in a last-ditch attempt to figure out how to do homework problems ("find an example that matches!").
In my statistics class that met this last September (ending last Friday), all of the students had a project; they chose a question, obtained data, and analyzed it. One student, Julian Seeman-Sterling, surveyed students at Quest to find out how much of the reading they did. Below are a couple of his results:
You can tell just looking at the histograms that there's no appreciable difference between the amount of reading that students claim to complete in the natural sciences as compared to other disciplines. And, indeed, Julian ran a statistical test on these, and there's no evidence of any difference. (Note that Julian calls "physical science" what is more commonly called "natural science"— that is, it includes things such as biology.)
I do have to say that I was surprised to hear that, but of course it all comes with caveats. These are the results of a survey of students at Quest. Quest is an unusual place; students only take one class at a time, and it's very intensive. They don't have stacks of reading for many different classes to do; they only have the one class. As such, they tend to be very engaged with the one class they are taking. Also, these are the results as reported on the survey. As Julian pointed out during his presentation in class, he couldn't know if they're really true without following a lot of students around throughout their day... and that wouldn't be entirely practical.
So, do students do less of their reading in physics and astronomy than they do in their humanities courses? I don't know. Julian's data suggests that that is not the case at least at Quest.