Pressure from natural overpopulation is thought to be a reason for large animals on islands to get smaller. The pygmy elephant from Java being an example.
Another reason for pressure from over population is destruction of habitat. Either affects a species the same way as it is the ratio that describes the pressure.
On the other hand, being freed from competition or predation seems to give some species an opportunity to get larger. Rats on islands (sometimes the same islands as where the pygmy elephant evolved) often get bigger. The Moa of New Zealand may be an example of this also.
It is hard to predict what will happen in any particular event. It is easy to backtrack that big rats evolved from smaller rats, or small elephants evolved from normal elephants.
One might even conclude that the inability to predict is an indication of a lack of scientific support for the reasoning. Yes, both environmental pressure theories seem logical, but that doesn't make them true. When you have a logical theory to cover every base you can't go wrong. If they get bigger, the theory works. If they get smaller the other theory works.
What if they changed size for a different reason, but we never explored it because we already accept the catch all theories. The presumption is that they had to change for the better, but we never assume that extinctions were the result of evolution gone awry.
Yes they are insightful theories and might be correct, but I always question theories that can't be disproven.