[FillardMillmore]

I tend to write longer e-mails - but I agree with you, long-form and short-form e-mails are both optimal for different situations.

The problem seems to be that it's a judgement call - and I'm not exactly sure of all factors/variables that are involved in that judgement call. The subject at hand, the audience, the depth of the e-mail thread to which you're replying, time-sensitivity of the subject, etc. - they all play a role. But my default position tends to be that I'll save time explaining in future e-mails by providing all relevant details at hand now.