Just out of curiosity, how many channels are you subscribed to? Also how many messages per hour? more or less...
14 channel, I don't know how many there are on the server. Not sure on the number of messages / hour, but it is quite a lot. Leaving slack on overnight has my numbers up to about 800MB. Regardless of how many messages, 800MB for a chat client is nuts.
[0] http://imgur.com/S35NWij