HIV is prone to mutations. Reverse transcriptase, which enables the virus to copy RNA back into DNA, is very error prone. This means that HIV has a high mutation rate, but because the virus is small, it can survive with such a high mutation rate.
The shorter the genome, the greater the probability that a sloppy copying process will be able to generate a copy without too many errors. (If you're a bad typist, you're more likely to correctly spell a short word than a long one, just because you have more chances to get something wrong in a long word.)
And even though most mutations are deleterious, so many HIV particles exist that an advantageous mutation (one that makes the virus resistant to a drug, for example) is likely to occur by chance.
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