Exchange ActiveSync 12.1 (introduced with Exchange Server 2007 SP1, released in Nov 29 2007) added a policy called mail body truncation size (see http://en.wikipedia.org/wiki/Exchange_ActiveSync#Exchange_ActiveSync_12.1).
This is a server-side policy that can be enforced for a group of users, as stated in the Microsoft Exchange documentation (see http://technet.microsoft.com/en-us/library/bb123484%28v=exchg.80%29.aspx):
Exchange ActiveSync mailbox policies let you apply a common set of policy or security settings to a user or group of users.
Maximum HTML e-mail body truncation size
This setting specifies the size beyond which HTML-formatted e-mail
messages are truncated when they are synchronized to the device. The
value is specified in kilobytes (KB).
Maximum e-mail body truncation size
This setting specifies the size beyond which e-mail messages are
truncated when they are synchronized to the device. The value is
specified in kilobytes (KB).
IBM Lotus Notes Traveler, which supports ActiveSync starting with version 8.5.1, has a similar setting in Default Preferences>Sync settings (see http://infolib.lotus.com/resources/domino/traveler/8.5.3/doc/ta853abd002/nl_be/NotesTraveler.html#Downloading_truncated_e-mail_message):
Email Body Truncation
Enables email body truncation. Characters beyond the default character
value in the email body are truncated from the email body.
So it looks like it is a bandwidth-saving setting on the server and you basically can't do anything about it.