Increasing the grain bill won't increase your efficiency. Given the same process it'll actually lower it. However, it add more fermentables to OVERCOME the efficiency decrease. Basically, it's not increasing lower efficiency, but rather compensating for it.
The easiest way to think of efficiency is to think of "mash" efficiency and not "brewhouse" efficiency. Mash efficiency is strictly how efficient you are at pulling sugar from the grains. When you look at brewhouse efficiency, you not only look at efficiency of pulling sugar from grains, but you also factor in losses to trub, to tubing, to hops, to your chiller, etc, and that can end up changing vastly depending on just different recipes. The problem is that BeerSmith defaults to Brewhouse Efficiency unless you tweak the settings, and if you don't get every setting just right, the numbers it spits out can be wrong. So it's possible (without running numbers myself) that you're actually getting everything right for your system, but BeerSmith is spitting out a difference estimate because there's an equipment setting off somewhere. Basically in your equipment settings (if you do it in a specific recipe it won't work for the equipment profile for future recipes, but if you change the equipment profile it may not change the current recipes, just be aware of that), zero out all losses (trub, chiller, yeast, etc), and set your batch size to be the post-chill volume, and then play with the pre-boil to be exactly what you need given your boiloff rate + cooling shrinkage. And then make your batch size the post-boil, post-chill volume, and NOT the fermenter volume. It defaults to fermenter volume, and if you have losses to trub factored into the equipment profile, then the efficiency numbers get whacky. But doing it this way, the mash efficiency and brewhouse efficiency become the same thing. And then if you normally lose X amount to chiller and tubing and what not, factor that in in your head, don't have BeerSmith do it.
Once you know the actual numbers are lining up properly (and it's actually your efficiency that's low, and not just your equipment settings), as far as how to RAISE your mash efficiency, I would look to:
-grain crush, as mentioned above. Generally a finer crush yields higher efficiency, up until the crush is fine enough that lautering becomes difficult and you either get stuck sparges or your efficinecy starts to go back down. If you're not crushing your own grain, good luck EVER getting consistent efficiency unless you bring a set of feeler guages to your LHBS, and mill the grain yourself adjusting the gap in the mill the same way every time. Easier to just buy your own mill. Plus you can buy in bulk that way.
-Lautering. Are you batch sparging or fly sparging? Fly sparging without the right equipment configuration can lend terrible efficiency, so try batch sparging and see if that helps. Better yet, split your batch sparging into two steps, and you'll do even better. If you insist on fly sparging, decrease your flow rate down to a trickle - sparging a 5 gallon batch should take no less than an hour fly sparging, and invest in a false bottom if you haven't already. Other designs (especially using a braided hose) are recipes for channeling and terrible fly sparging efficiency.
-Do a mash-out. In addition to cementing fermentability by denaturing enzymes, it'll also make the sugars slighly more soluble. It's not a dramatic increase, but I get a couple points extra efficiency if I do a mashout.
-Dial in your mash and sparge pHs. I'm not sure what the mechanism here is, but I know that having a mash pH out of range can hurt your efficiency.
