 What is the most likely reason that creatures on the alien planet have grown to such a large size?
The ice-cap, by its existence and circular shape, proved that the planet rotated at a not unreasonable rate. Its size, too, told about temperature-distribution on the planet. The fact was verified by the thick, dense cloud-masses which covered most of the surface,—all the surface, in fact, outside the ice-cap. The process would be to find a planet and check its climate and relationship to other planets, and its flora and fauna against descriptions in the Directory. All five of its proper company watched as the planet's surface enlarged. If somebody were wrecked on an even possibly habitable planet, the especially developed seed-strains would provide food in a minimum of time.
 The planet, like many others, is being used as a site to copy a habitable eco-system, but has been left unchecked.
 The planet is being used by the government as a site to breed creatures that could keep the population in check.  The planet's atmosphere comprises gases that target the DNA of living creatures, causing them to grow in size.  The planet, like many others, is being used as a site to copy a habitable eco-system, but has been left unchecked.  The planet's cheesy, perforated ground is made up of a substance that causes living creatures to mutate.