In this case at least, the maximum file size varies with your plan.
So for example I connected my twitter and my github, so if I tweet (or someone tweets @me) it shows up there. Similarly if I commit stuff to github (or presumably if someone on my team commits stuff) it posts the commit message to me.
It's pretty neat, might dig in a bit more later.
[1] http://www.pcworld.com/article/2018603/get-a-free-25gb-box-a... [2] http://hojoki.com/#plans
Moreover, if somebody uses even 10GB, they're pretty well locked in to box.com. That's more than any competitor I know of gives with free accounts, so to switch, you'd be looking at paying, right off the bat, for service you haven't vetted yet.
So while 25GB may feel generous, it's really only a one dollar loss leader, and the "catch" is built in to ordinary usage of the service.