Per the devs:
"Burgling a large money sack will either give 2 coins or 1 lantern, with about a 25% chance of getting 1 lantern. Small sack always gives 1 coin. THere's no way to know what you'll get from a large sack ahead of time."
I think I've received nothing, at times, when burgling a dark building (which means it hasn't be burgled, yet). So I'm not certain dark buildings always have a large money sack waiting, or if there's a chance it contains nothing.