Yeah, safe capacity is a bit like tent capacity.. There may physically be the volume to hold that number but you wouldn't want to try it.
I think I have well under half my safe's capacity and it seems "full".
Virtually all of the "quick" methods of people breaking into substantial safes seem to be related to pushing it over onto its back, so making it difficult to move, and in a space where getting pry bars etc to it is difficult will help.
One of the things I came to figure out with secure storage is that anything can be broken into eventually, so what you're doing is delaying that. A "stack-on" type might take 30 sec to break into, a low-end "vault" type (what I have) maybe a couple of minutes if someone knows what they're doing, and a high end vault may take several minutes or more. Considering "vault" type safes run from something like I have for <$500 to $20k+ there's a big range.
Now considering the above the question is who are you trying to keep out and how long do you need to delay them?
In my case
A) I have an ADT system, so hopefully someone breaking into the home triggers the alarm, at which point I think there's 30 or 60 seconds to disarm before the alarm actually sounds. By the time alarm sounds, ADT calls to check on me, calls the local PD etc and so on, probably at least 5-10min best case for response, so a criminal might have 5-10min "undisturbed" in my home.
Since I'm not expecting Danny Ocean to be planning the break-in I figure once the alarm is sounded most common burglars would look around and grab what's quick and easy.. TV, XBOX, etc.
So I also have safe(s) in somewhat concealed, non-conventional places
C) Hope for the best but plan for the worst.. make sure you have an inventory with serial #'s in case anything does get stolen, you can properly report it.
Also, like most others have said I do not store ammo with the guns, except what's in the magazines for my "carry" guns. Ammo is in a whole separate area of the house, again somewhat concealed.