In answer to Matthew's question, no, FedEx does not operate B767s. The only "original" Boeing equipment they fly is the B727. However, one could probably look at an A300 or A310 freighter (both of which FDX operates in abundance) and mistake it for a B767 on first glance. Having never been to Orange County, I can't say for certain what FDX flies there, but the 'bus would be my guess.
As to MDW (and most short-runway airports) landing there isn't the problem. Most large aircraft can land a lot of places they can't get out of (legally, anyway). I suspect that the B777 could probably be stopped in the length of MDW's runways, but it certainly couldn't get back out (except perhaps empty and with minimum fuel, even then probably close). The other issue is pavement strength. I'm fairly certain the pavement (rwy, twy, etc) at MDW isn't stressed to 550,000 lbs or thereabouts, as would be required for the 777. I would guess the MGTOW of the 757-300 is probably around 300,000 lbs ... I know the -200 is 255,000, but I don't know how much higher the -300 is.
We even have this issue in the King Air. We can land and stop almost anywhere (I've stopped the airplane in 2000' without braking, just using moderate reverse), but getting out with an acceptable safety margin requires 4000' or so. Really set off a former employee when we told him we couldn't take him to a 3000' runway somewhere.
FL270