I don't know enough about medical statistics to say, but I often see small sample sizes in studies where the effect size is expected to be high. That may be the case here.

There are too many variables in diet. If they study steak every meal vs rice and beans every meal they can come up with one. However most people are not that one-tracked either way. Sometimes the rich eat rice and beans, sometimes even poor manage to afford steak. For steak, did I mean beef, lamb, goat, pork... - this might or might not matter. There is also chicken, turkey, snake, deer, elk - and dozens more animals people eat which might or might not be healthy. OF each of the above there are different cuts (does it matter?), different fat levels (does it matter?). And that is just meat, how many varieties of beans are there, what about rice? What about all the other things people eat?

the effect size will not be high