I thought that 'si' meant himself, herself, oneself .... Is 'si' a stand-in for 'we'as well?
Si has two main purposes: one is reflexive, and the other is impersonal. In this case it's the latter, "one can't carry more" or "you can't carry any more"; they often don't sound as good in English and using a passive form is better.
Thanks, but why exactly is "He cannot carry any more" wrong? Would that be just "Non può portare di più"?