This is a very difficult question to answer for two reasons: people's college experiences vary significantly, and people's learning styles and medical schools' teaching styles also vary significantly. So with the caveats that I'm speaking only for myself, and also that I've only finished my fifth week of medical school, I would say that so far at least, medical school has not been "harder" than college was in terms of the level of conceptual difficulty. But it is definitely more challenging than college, for at least two reasons.
One thing that makes medical school more challenging compared to college is that the pace in med school is a lot faster. Topics that might take you several weeks to cover in college get covered in med school in a day or two. We are doing our entire biochemistry/molecular biology unit in 10 weeks, for example, which is way less than even a single college semester. (That's why college biochem is a pre-req for this program!) Plus, in med school, you have to do a lot more of your learning independently, especially in a program like this. There aren't any teachers here to spoonfeed this information to you in lectures; you are responsible for doing the reading and seeking out whatever extra help that you need on your own. Overall, I think that it's great to be able to learn things at my own pace, but it's a little scary sometimes too, because you still have to keep up with the class's pace. For that reason, I study every single day here, whereas in college I could get away with not studying every day for a lot of my classes, and for some of my college classes I'd even only study the day before the exam. It's very clear to me already though that the procrastinate-and-cram strategy so many of us used to get through college won't work very well at CCLCM.
The second thing that makes medical school more challenging than college is that your classmates are a lot brighter and more motivated on average in medical school. A lot of people, myself included, were used to being the big fish in the little pond in college. Many if not most of us were the top students in our classes. So now all of a sudden the bar is raised, because basically everyone here is really smart and a really good student, or they wouldn't be here. I think you will find that to be true at any medical school around the country, but considering that we have been working together in small groups to solve problems here at CCLCM, I am constantly being made aware of how much I don't know that other people in my class do! It can be a little intimidating to be in this kind of high-intensity environment sometimes. But I think that overall it is to my advantage to be going to school with so many highly accomplished people, because I am learning a lot from my classmates and not just from my books and the faculty.