The limitation comes from the lens resolution. There's a point where more megapixels simply becomes a burden without bringing any perceivable benefit at all. I personally believe with 35mm, it is probably around 20 megapixels. Of course, there is not a clear cut off line, but a gradual approximation.
With 35mm full frame, the first critical point of megapixel arrives at 4M. I will show you why below. Before reaching that point, any increase of number of megapixel resulting a quite strict one-to-one increase of resolution. For example, 2M quite literally has twice as much much resolution as 1M. After that, this will start to degrade. For the