At my school the first year was a total of 36 credit hours and the second year is a total of 38.5 credit hours. These numbers do not correspond to undergraduate credit hours in difficulty though.
Basically every test in medical school feels like a semester final and covers about as much information.
As has been said many times before the information is not hard at all to grasp, but it's just the volume of memorization.
For example I just took a microbiology test and here is a sampling of all the "alphabet soup" of growth factors, proteins, and cytokines we had to keep straight:
C1qr2s2, C4, C4a, C4b, C2, C2a, C2b, C3, C3a, C3b, C5, C5a, C5b, C6, C7, C8, C9, Factor B, Factor D, Factor P, C1 INH, DAF, CR1, C4 BP, Factor I, TNF-alpha, Fas, FasL, Th1, Th2, IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-8, IL-12, IL-13, IgG, IgA, IgM, IgE, IgD, CD 40, CD 25, CD 28, B7, IFN-alpha, IFN-beta, IFN-gamma, GM-CSF, MHC-I, MHC-II, HLA-A, HLA-B, HLA-C, HLA-DP, HLA-DQ, HLA-DR, LPS, LD3, CD 14, TLR 2, TLR 3, TLR 4, ICAM, VCAM, CD 8+, CD 4+, LTC4, LTD4, LTE4, H1, H2, H3, etc, etc. (and there are many, many more)
Then add on top of that a big list of antibiotic drugs. It's doable but the memorization of medical school sucks.
So far 2nd year has been much worse than 1st year, you just get better at studying (or in my case cramming) and it doesn't SEEM as bad.
edit: This semester I have Pathophysiology I, Microbiology, ICM-2, Genetics, and Ethics.