In physics at least, isn't that because most of the low-hanging (low-energy..) fruit is gone? CMS or Atlas at the Large Hadron Collider are feats of engineering, not to mention the physics, and doing all of that takes a large organization. In theoretical physics, people seem to still be doing reasonably well with the lone-genius thing (although I suppose Peter Woit disagrees).
Biology is interesting because small, high-impact groups become arguably more sustainable every year due to cheaper, smaller instruments and the rise of contract services (e.g. DNA sequencing). On the other hand, many of the human problems biology is attempting to address are hard to inferentially study on a small-sample level, again necessitating large organizations.
