If the head fits, the mouse fits
How Does a Mouse Fit Through a Gap?
A mouse can push through a hole the size of a pencil, because the widest part of it is the skull — and the skull is small.
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Step 1 of 5
The whiskers measure first.
A mouse sweeps its whiskers across a gap before committing. Each whisker sits in a socket packed with nerves, so the mouse gets a genuine measurement rather than a guess.
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Step 2 of 5
The head is the test.
A skull is solid bone and cannot squash. Everything behind it can. So the rule for a mouse is simple: if the head goes through, the rest will follow.
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Step 3 of 5
There is no bone holding the shoulders apart.
Yours is a strut across the chest, and it is why you could not follow Your collarbone is a strut that fixes how close your shoulders can come. A mouse's is tiny and floating, so its shoulders slide inwards and the chest narrows.
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Step 4 of 5
Then it pushes and flattens.
With the head through, the mouse braces its back legs and drives forward. The ribs flex, the body squeezes to the width of the skull and it pours through the hole.
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Step 5 of 5
Small means always eating.
A small body loses heat fast because it has a lot of surface for very little inside. A mouse has to eat something like a fifth of its own weight every day just to stay warm.
Mice cut their whiskers back on each other during grooming. A mouse whose whiskers have been trimmed by a cage mate is noticeably worse at judging gaps.
The short version
A mouse fits through a gap the width of its skull, because nothing behind the head is locked at a fixed width.
Try it yourself
Measure the gap under a door. A pencil-width is enough for a mouse, which is why draught excluders are also mouse excluders.