Most medical studies have a small sample size because it is difficult and often prohibitively expensive to do studies with large numbers of subjects. Small sample size increases the likelihood of spurious results.
The other problem is that mainstream journalism does an abyssmal job of reporting science. How many times have your read a newspaper article or seen a segment on the evening news that tells you that X is good for your health or X is bad for your health as if it was an absolute truth, a law of nature on par with V=IR or F=ma? Then, out of curiosity, you look up the actual journal article published by the scientists and find their claims to be considerably more modest.