Short answer to this question:
- Don't merely provide a link to an answer because if the link breaks the answer will be worthless.
- You can add a summary to your longer answers (like I've done here).
- Do revise and shorten your own answers when appropriate, but don't do this with other people's answers.
Long answer to this question:
Short answers are great, but may not cover all the important aspects of a question. The more difficult the question and the more factors are involved, the longer a good answer will be.
Of course there is a limit. A question can be too complex or broad to discuss properly in a limited amount of words, but those questions should be closed as being too broad.
What I myself sometimes do when I've written a long answer, is to start the answer with a brief summary with the main points of my answer. This way the OP can have his/her detailed answer and other less-interested readers can use that summary to see the main points and then decide whether they are interesting enough to read the full answer.
I think you should be careful with your suggestion to shorten answers and mainly refer to other references. Links tend to break over time and we want an answer to be valuable for a long time. If the linked webpage or document is gone (parts of) an answer can become worthless. That is why it's policy on all StackExchange sites that link-answers are not real answers and why we should encourage users to summarize the contents of a linked page in their own words. The linked page should merely be provided for extra detail for interested readers.
When it comes to other references such as books, the same does apply. A book doesn't go missing like a link, but it may be difficult for people to find the referenced book.
Finally, with respect to doing clean-up; I think it's great if you revise and improve your own answers from time to time, but I don't think it is a good idea to completely revise other people's answers. Small improvements are ok of course, but removing parts of an answer because you yourself feel it is long is a big no.