The longest email may be trying to prevent a disagreement that needs an actual conversation.

Length is often armour. Anticipating every objection, pre-answering every question, arranging the reasoning so tightly that nothing can be pushed back on: all of that is an attempt to win the exchange before it starts, usually because the writer is worried about how it will go. The result is a document nobody can reply to properly. What was meant to prevent conflict tends to postpone it into something colder.

When you notice a message getting long, stop and ask what you are defending against. Then send four sentences proposing a call, and have the difficult part out loud.