Regarding the gap-plugged comment, the ilb's in a 3/4 are typically bigger since they tend to have to take on a guard or tackle more frequently than a traditional 4/3 inside backer. I'm not sure he'd last long if he could only get to 225lbs or thereabouts.
But I do agree that the ilb position does need someone who can really jet.
This 3-4 (really a 5-2) is designed to keep the ILBs clean so they can run around and make plays.