D linemen for the 3-4 don't necessarily have to be big depending on your scheme. If you're running a two-gap scheme, they do have to be big to keep from being pushed out of position. A one-gap scheme that relies on quick penetration by the D line with the LBs filling the gaps that open up, you don't necessarily need 300lb+ guys, even at nose tackle. In that case, you need quick, strong guys who can get off blocks fast and make plays in the backfield.
It's all complementary. The fundamental philosophy of the D line is complemented by the linebackers to cover weaknesses. And that's not even including safety help if they read run.
I've always liked the 3-4, except for the fact that your OLBs need to be extremely good. On top of having to be superb athletes, they also have to be both skilled in coverage, pass rushing, and run fitting, but also have to have lightning quick decision making. If the OLB in a 3-4 gets out of position, it'll throw the whole scheme off.
That said, 3-4 meshes well with pretty much any kind of secondary coverage (I absolutely love it with press quarters) and it's the best scheme to zone blitz out of. Even with the personnel we have on hand, I could see the Davis twins being good at either nose or DE in a 3-4, especially with the fact that Carlos has shown flashes of ability to get off blocks and get into the backfield. If Kalil has the same kind of ability, we will be in good shape with them and Stoltenberg. M. Newby is almost prototypical for the OLB position in the 3-4, and Alex Davis looks like he fits the mold as well. A press zone 3 high coverage with a corner blitz off the edge and an OLB blitz from the other side could be brutal with who we have on hand. Kalu blitzes well from the corner position.
If we can pull one or two tall, fast OLBs from JUCO before signing day and maybe one more nose tackle with good skills to get off blocks, we will be in good shape if we do more to a 3-4.
Of course, since I'm biased towards mY favorite scheme, take what I say with a grain of salt.