Throughout the novel, Maxim is shown to have an almost superhuman physical condition, easily performing unthinkable feats, which include, but aren't limited to
Being able to run for a very long time without fatigue
Being able to carry a (heavy) armed man while running
Reflexes considerably faster than those of the Inhabitants of Saraksh, able to dispatch ~10 men within a few seconds
Incredible resistance to radiation - able to swim in a radioactive river, and eat a radioactive fish with no harmful consequences
Able to survive 7 (?) bullets from a very close range directly to his torso; at least 2 of them should be fatal as per an assessment given later by a doctor
The list may be amended, but this is the general shape of things. The novel is set in the 22nd century, and Maxim implies all humans are considerably improved physically by that time (he also states later in the novel that prolonged exposure to the climate of Saraksh has weakened him).
However, the novella Space Mowgli (aka The Kid, "Малыш") is set in the same time period (in-universe), and its characters do not seem to be as developed physically as Maxim (if they were, they wouldn't be as challenged when playing with the Kid), which leaves two possibilities:
Only the Free Search Group members are as physically developed, possibly due to some tampering with their bodies and stuff (for which there's no evidence in what I've read so far).
It's an inconsistency in the writing, because Prisoners of Power and Space Mowgli were written separately in different years (were they? I'm not sure).
How to understand this?