"As with any safe, you really need to have it bolted down or secured somehow. "

Even heavy ones get stolen. A decade ago a thief heard a guy had a load of money in a large and heavy safe. He used a sledgehammer on the front door walked in with a logging chain, tied it around the safe, slipped it over his trailer hitch on the way out and took off in his truck. The safe missed the door a bit, took out a good bit of wall, and bounced along behind the truck until he got it to where he could open it. Where he used a torch and jackhammer to break into it.

He got about $50 and ten years in jail because the sheriff deputy followed the skid marks and divots dug out by the safe to the man's wrecking yard, with his name on the gate, and found the destroyed safe in plain view.

He was a bold and effective thief. But one that wasn't particularly smart about covering his tracks.

A lot of people have found the most cost-effective safe to be be one concreted into a slab. These are very difficult to remove and hard to break into if place in a tight spot. The safe is slightly below the slab so they enjoy a certain amount of protection from fire even without an actual fire rating. Their main weakness is that water might get down into the safe. Many of these units have rim that sticks above the floor several inches and that eliminates a lot of the danger from minor flooding.