Am I beholden to a professor that helped my application - would it reflect poorly on me if I don't join that professor's group? I have just joined a PhD program in the US that I am very excited about. I was accepted despite not having the top grades usually required, in part thanks to good results on my GRE, and in part thanks to a discussion with a professor within the university, who gave me an informal interview and put in a good word for me (the professor is not part of the selection committee). He is all but ready to offer me a spot in his research group, but I would like to have a look at different research groups that might be more in tune with my expectations for my PhD. His group is fairly tiny (as an aside, from your experience, does group size have an influence on the quality of research?), and the research seems quite number-crunching-heavy and I was hoping for more experimentation (we're not in CS).

On one hand, I am very grateful for the opportunity I am given; on the other hand I want to make the most of that opportunity and it could mean not joining that professor's group. Essentially, am I acting spoiled by not automatically accepting that proffered hand?
Assuming you've been accepted by the university and the department (and have it in writing, i.e. an acceptance letter) -- the short answer is no, you aren't beholden to them.

The longer answer is that while you aren't beholden to them, it sounds like they already expect that you'll be joining them. How to approach this depends more on the faculty member than anything else. Some faculty members, when told that you "want to shop around and see what the university has to offer", will gladly nod and tell you that makes perfect sense, and they're hoping you'll still join them. Others may consider this a "rejection" of their help, or even a betrayal, and thus your bridge with that faculty member is burned forever.

Here's the trick: you want to avoid the latter category of advisors anyway. So tell this potential advisor the truth, see how they react, and best of luck at your new university!