I'll write more later, but one of the keys to "deep" I've found was the psoas - the diaphragm interacts with it towards the central inferior aspect. start your inhale there, let the psoas feel like its dropping a bit as you expand the abdomen and let the inhale gently roll forward and upwards along the diaphragm. also let the breath start from there = dont use your air passageways to draw in air, let the draw come from the bottom.
with respect to increasing total overall volume - when doing EB its not usually a focus to increase to depth - you can do depth increasing exercises any time, really. BK Frantzis recommends a 70% rule with regard to training - his stuff is very water-method focused and gentle, so you can take that into consideration. but for a calm, soft, slender, deep breath, when beginning EB I found it quite beneficial to slightly exaggerate abdominal motion to help facilitate getting the breath all
there as opposed to "breathing from the nostrils" or throat or what not. anyway, your lungs are like a balloon and can expand significantly beyond the chest capacity that you have and at some point in the breath you will be putting an increasing energetic component into simply providing the extra lung stability (the same goes at a more subtle level with the air through your breath passageways - more air flow = more air pressure = more energy input into "maintaining structural integrity." and in lessening that aspect, more subtle still is the little bits of air turbulence that also rob energy from the breath via vortices that form because the breath is moving fast enough - do eb and get it slow enough and at some point even the breath turbulence disappears, and this is very cool because you cant feel yourself breathe when you get to that point!) so the general 70% rule will allow for pretty much the most energetically optimal breath where physical components are at their "least level of energy consumption." that's part of the main focus - get all of your physical processes into a low-energy state and keep your awareness centered at the dantien and that will generate more energy, and when your body has all this extra energy and you're not doing anything with it, it will automatically put those resources to good use in the direction of core processes.
another of my teachers always says to do "the one breath" which has this congruence with EB; part of the process of eb and getting it all smooth is to work on the transitions between inhale and exhale and make those smooth enough that you have this rolling transition from one to the other. start rolling between the two and your mind doesnt distinguish the difference quite so much - realizing my focus, at some of my more profound moments - hehe aka the first spark of the mind's distraction after some indeterminately long moment - I realize I've only been moving my abdomen with my awareness focused there and very little else, shining peacefully