I'm having some difficulty reproducing this consistently. Here's what I've been able to find so far:
1) Files sent from internal emails seem to come through regardless of size
2) A large file (>25MB) sent from outside did not come through and the sender did not receive a message delivery error - it just disappeared into the ether
3) A less large file (15MB) came through into everything (OWA, PC, and Mac).
So, I'm now at a loss to explain the handful of emails where it has occurred (i.e. emails that came through into OWA but were nowhere to be found in Outlook 2011).
I'll post back here again if I can reproduce the problem consistently.
Thanks!