ot: data consumption IMAP vs POP

I have a domain with about 50 mailboxes, server is located here in AUS but most of the users are on a LAN is SEAsia location. They were using POP (with Thunderbird), I suggested then can use IMAP instead, so they did.

now they are asking;

"Looks like Imap is adding a lot to our internet bandwidth"

I guess they have some bandwidth limitation on their link

I think I can understand that IMAP would increase bandwidth requirement, didn't expect it to cause 'problems'

is there any optimization or changes I can make to reduce that ? the b/w limitation are at the client LAN link

any other suggestions ?

thanks, V