Because klein is the adjective describing the subject after the linking verb is.
This works very much like English:
So the pattern is:
subject + is + adjective
In this sentence, klein is being used after the verb is, so it stays in its basic form.
Afrikaans does not add any agreement ending here for number or gender. So whether the subject is singular or plural, klein stays klein in this kind of sentence.
For example:
A rough English-friendly guide is:
A couple of notes:
These are only approximations, but they are good enough to get started.