This isn't dealt with in either the film, nor the film's original script. The process itself is simply (and intentionally) hand-waved in Dr. Asbjørnsen's speech at the start of the film.
DR. ASBJØRNSEN: ... my team and I discovered a process by which all organic material can be reduced at the cellular level by a ratio
of approximately 2,744 to one. Such a process converts a man of 1.8
meters to just 12.9 centimeters. A wide array of flora and fauna were
subjected to this process and, with the exception of some fish and
shellfish, absolutely no side effects were detected. Please lights.
The room dims. A GROUP PORTRAIT of the gathering at the Asbjørnsen
home appears on a screen above the stage -- the first image in a
DR. ASBJØRNSEN (CONT’D): Once the safety of the procedure was confirmed, thirty-six brave volunteers joined my wife Anne-Helene and
me as the very first humans to undergo cellular reduction.
A NAKED SUBJECT poses before reduction, arms extended like Da Vinci’s
Vitruvian man. Next he shows off his new TINY BODY beside a metric a
ruler held by enormous fingers.
DR. ASBJØRNSEN (CONT'D): The exact science behind reduction itself falls outside of our discussion today....