What affects front to back depth in room/ system?

It’s all about setup. There are several excellent books on setup. Buy one, read it , learn how to setup your room

Note that unless you've never actually owned a stereo system or you have no faith in your own taste, the proper setup is what sounds best to YOU.

I went on the search for greater depth and have concluded it is a bit of a myth.  My speakers are 4' off front wall and 3' off sidewall.  My side reflections are treated. I have a fabulous sound stage and very holographic sounds.  There is depth but not to the degree some describe (i.e. behind and outside the front wall.).  

While it sounds counterintuitive, side wall reflections impact the width and depth of your sound stage more than I ever imagined.

As you consider room treatments, read a few articles written by Art Noxon, owner of ASC (Acoustical Sciences Corporation) Tube Traps. 
 

I like his tube traps because you can literally “dial in” the soundstage by simply adjusting the amount of diffusion and absorption you need. 
 

Use a mirror to find your 1st and 2nd side reflections, and watch your soundstage adjust to meet your needs. 
 

Hope this helps.

labpro is correct that sidewall reflections affect image depth...all other things being equal, by moving speakers closer together you can increase soundstage depth but at some point it's at the expense of sounstage width. It's an iterative process, and almost every room is different...