 explained in-game. In the Super Metroid comic, Armstrong Houston suggests that the creature was heavily exposed to Beta-Rays which resulted in its unnatural size increase; since the baby was the only existing Metroid during Super Metroid 's events, it is probable that the Space Pirates were forced to use an excessive amount of beta radiation to produce a satisfying amount of Metroids, thus mutating it. This would also explain why the Metroids featured in Samus's Zero Mission did not turn into Big Metroids despite exposure to beta rays. However, since it is stated in Other M that only special Metroids have the specific genetic coding to become Queen Metroids and the Queen featured in the same game was created from the baby's DNA, it is possible that the Big Metroid was a stage in the baby's evolution into a Queen.
The battle against the Big Metroid is scripted in the game. As mentioned before, it is invulnerable to all of Samus' weapons and it is programmed to chase her at an incredible speed and weigh her down as it drains energy off of her, preventing her from leaving the room where it is "fought". It cannot kill Samus, since it is also programmed to stop exactly when one unit of energy remains, triggering the cutscene where the Big Metroid recognizes Samus. However, it is nevertheless possible to "defeat" this creature: Samus can successfully evade the Big Metroid's persistent chase and leave the room using quick maneuvering of her jumping abilities without triggering the mentioned cutscene (a popular tactic among speedrunners). However, if the Metroid is latched onto Samus as she exits the room, she will be glitched in the form of incredible slowdown in her movements, exactly as if the creature was still weighing her down. Another glitch that can result from having the Big Metroid latch onto Samus before leaving its room is the heroine being frozen in her exhausted animation (hit by the Laser Brain Attack and her near-complete energy absorption), unable to recover. Therefore, these glitches force players to reset their consoles.
"There's nothing you can do to get it off of you."
- Matt Manchester, an environment artist for Metroid Prime 3: Corruption, wrote design notes explaining why the Metroid increases in size, but they are not considered canon as they did not make it into the final game due to text size limitations: "Observations of captive Metroids display a rare anomaly in their developmental growth. Approximately .08% of contained specimens never reach the biological process that leads to their otherwise natural metamorphic stages of maturity. While developmental stages are stunted, physical growth continues at a congruent rate. Efficiency of energy consumption increases steadily as well. During behavioral analysis, greater intelligence and problem solving skills are observed. Heightened abilities of energy consumption make this anomaly a potential key in isolating and duplicating their feeding process artificially." In his notes, he considered placing a Big Metroid in a containment in Metroid Processing, though this never came to fruition: "In one chamber, I wanted to put a fat giant metroid, like in the end of Super, and then tie in an explanation as to why some Metroids (like the one in Super) get really big in their larval state, but others evolve into more adult forms. Again, none of this is canon by any means, but just the fiction I wrote for myself when making the room."
- Interestingly enough, the Omega Metroid seen at the end of Metroid Fusion also suffered from an unnatural growth. Since this Metroid was also cloned from the baby like the Queen Metroid in Other M, it may also have been transforming into a Queen. However, this was never confirmed.
- The first encounter with the Big Metroid is accompanied by the boss theme heard during the fights with Kraid, Crocomire and Phantoon. Also, the ominous music that plays directly before battling both Kraid and Phantoon is also heard prior to fighting the mutated Metroid. This, coupled with the fact that the creature can be "defeated", constitutes the Big M