1. The population is under selection pressure from predators
2. Hey now, no population is ever at H-W equilibrium:
mutations happen
immigration and emigration can occur
the population is not huge, which means that genetic drift can happen
mating is not completely random -- a rat is more likely to mate with someone local than with someone living in Paris, France.
b) The small rats will be selected for (assuming there are predators in this ecosystem that are less likely to look in bushes, and further assuming that the small rats do not have reduced fertility by dint of being small)
This is "directional selection"
c) I would expect the "smallness" allele frequency to increase in this population over time, and the "normal size" allele frequency to decrease.