HD's are hard to achieve, no mistake about it, thats why they're high distinctions afterall. Personally, i reckon they may be harder or easier depending on the subject to 40's in VCE. The marking varies heaps by subject and how tough the content is obviously varies. Mix this recipe in with the fact your likes and dislikes will be different and boom, you get a heap of variability. Uni is (mostly) work in = work out, especially until you reach the higher end of marks. Below say 80ish, its definitely work in, work out. The amount of marks i've dropped by simply being late or forgetting to do quizes is staggering (yeah, i know i know, bad habit). If you do your work on time and with decent effort, theres no reason to suspect for most subjects you'd score anything less than a credit (60) bare minimum. A lot of the marks are showing up/actually doing your work/doing it on time. With decent effort, theres no reason to suspect you couldn't get over distinction (70) either. Gets a bit more tough towards the higher end but if you work at it, definitely achievable.
The textbook is that size because (usually) it isn't tailored to your course. All VCE textbooks are written to exactly match the VCE curriculum. Many books in uni are used globally and are fairly general and broad, that might contribute to their thickness as well.