The Wikipedia article is probably a good place to start: https://en.wikipedia.org/wiki/Cryptographic_hash_function

Though I'd say this isn't something you read directly, but rather understand by going through cryptographic security as a whole.

To keep it short, cryptographic hashes make a few guarantees. A single bit change in the input will cause a drastic change in the output. Due to the birthday problem, the length needs to be double the length of a block cipher key to provide equivalent security. And a few others. When you chop it down, you potentially undermine all the security guarantees that academics worked very hard to analyze.

Even a small change would require going to a lot of work to make sure you didn't break something. And when you've read up on cryptography in general and understand it, this tends to be an automatic reflex.

None of which really matters. GP's big assumption is that the hash size grows with input size, which is not true. Hash size stays fixed no matter the input.