The best long documents could be shorter. The worst long documents are long because their authors confused length with thoroughness.
Length is easy to produce and looks like effort. Actual editing — the removal of everything that doesn't contribute — is harder and looks like less. A 10-page report that says what a 2-page report could have said is almost always worse than the 2-page report, not better, because the reader has to do more work to find the signal in the noise.
The rule I try to apply: every sentence should do one of the following — advance an argument, provide evidence, create context, or be explicitly transitional. If a sentence does none of these things, it should be removed. Filler sentences feel like they're adding atmosphere or signaling professionalism, but they're actually increasing the reader's cost of extracting the actual content.
This is harder in email than in any other medium, because email often requires calibrating to relationship and register in ways that add words without adding information. "I hope this finds you well" contains no information but it signals something about the relationship and the formality level. That signal may be worth the cost. It often isn't, but you have to think about it each time.
The principle generalizes: prose is a communication medium with a finite bandwidth and an uncertain channel. Every unnecessary word is channel noise — it reduces the signal-to-noise ratio and increases the work the reader has to do. Concision isn't rudeness; it's respect for the reader's time and attention, which are the real scarce resources.