In many years of fishkeeping I've never once seen a system what had a "lack of surface area" be the limiting factor on how much bacteria it housed...
...In every set up I have ever kept or was otherwise personally acquainted with, limited ammonia supply is what limited the quantity of bacteria... including bare bottomed tanks... including tanks without fancy Bio Media...
So the system will have a certain amount of bacteria... that amount will be dictated by the amount of ammonia produced... If there is gravel, it'll live in the gravel... if there is Bio Media, it will live in the Bio Media... If there is nothing but sponges for media and sand for substrate, it will just as happily live there...