question:  What is the most likely reason that creatures on the alien planet have grown to such a large size?
context: The ice-cap, by its existence and circular shape, proved that the planet rotated at a not unreasonable rate. A water-ice ice-cap said that there were no poisonous gases in the planet's atmosphere. Its size, too, told about temperature-distribution on the planet. A large cap would have meant a large area with arctic and sub-arctic temperatures, with small temperate and tropical climate-belts. A small one like this meant wide tropical and sub-tropical zones. 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. But since there were ice-caps there would be temperate regions. In short, the ice-cap proved that a man could endure the air and temperature conditions he would find. Two of the Nadine's four-man crew watched out the same ports as the planet seemed to approach. They didn't seem to resent what he'd tried to do, or that he'd brought them an indefinite distance in an indefinite direction from their last landing-point, and they had still to re-locate themselves. Without rigid control of space-travel, any criminal anywhere could escape the consequences of any crime simply by buying a ticket to another world. It could have travelled at almost any speed in practically any direction for a length of time that was at least indefinite. 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. If they left a beacon, they may have left an identification of the planet. All five of its proper company watched as the planet's surface enlarged. That will depend on where we are from Loris because we have to keep enough for ourselves. If somebody were wrecked on an even possibly habitable planet, the especially developed seed-strains would provide food in a minimum of time. But space-travel regulations are especially designed to prevent such escapes.
output:  The planet, like many others, is being used as a site to copy a habitable eco-system, but has been left unchecked.
options:  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.