No as I have 120G and 87g free. No long emails. Sometimes even real short ones. But none that I can figure out.Is your storage getting low on either device? Mail might be conserving space. Are these exceptionally long emails; like three or four screens worth, with lots of images?
That's pure guesswork. It's possible 8.3 is just more conservative with email length limits for initial download.
You might check to see if Load Remote Images is turned on in Settings > Mail, Contacts Calendars; not that I think it should have anything to do with the problem.