I have two medium sized safes.
One large one flat wouldn't go in my house- too much weight for the floor itself, not able to get down to the basement, I'm NOT leaving it in the garage (too much in there as it is). Two safes also allow me to secure my collection in difference places- one quicker to access, one elsewhere in the house. If a thief would break in, he would need to spend much time making noise to get into one of them, then do it all again in order to get into the other... something one isn't likely to do.
One bigger one would indeed be cheaper, but sometimes it just isn't possible or desirable to do that. If I were building a house from the get-go I'd be likely to harden an area of the basement for this purpose (concrete walls, a good strong door, etc.). But I'm not.