the first-ever Doctor Who Dalek story, er, The
Daleks, the First Doctor and Ian Chesterton
remove a Dalek mutant from its casing so that Ian can hide
whenever we have seen inside a Dalek casing in later stories,
such as The Five Doctors, Resurrection
of the Daleks and Dalek, all the excess
space surrounding the creature has been taken up by machinery.
So how can Ian fit inside?
Daleks have evolved over the years, not only in terms of the
technological development of their casings - such as the addition
of vertical slats, from The Chase onwards, and the
deployment of personal force fields, from Dalek onwards
- but also in terms of the mutation of their bodies. For example,
they have hands in The Daleks, whereas their only visible
appendages in The Five Doctors and Resurrection
of the Daleks are tentacles. The imperial Daleks possess
crustacean-like claws in Remembrance of the Daleks,
in which the Doctor remarks that, "They've mutated again,"
while the creatures seen in the new TV series have developed
a single eye that we've never seen before.
it's possible that the mutants seen in later series have changed
in size and/or need more equipment to support them or to carry
out additional functions. It may be that the travel machine
seen in The Daleks has plenty of room for Ian to get
inside, even though he wouldn't have been able to do the same
with casings seen in subsequent stories.
An alternative, and far simpler, explanation would be to assume
that the Doctor and Ian remove some excess equipment before
Ian climbs inside. While the camera is on Barbara and Susan,
who are guarding the corridor outside, we do not see what
the Doctor and Ian do between removing the mutant and Ian
entering the machine.