Lots of people can do ab wheel rollouts from the knees. The problem comes when you lengthen the lever arm and do rollouts from the toes. Taller guys really have a hard time with this.

Anti-extension strength largely depends on the size of the lever arm you create, assuming the trunk serves as the fulcrum. That's why you see 5'5" guys who can ace this movement and 6'8" guys who just plain suck at it.

To get the benefits of long-lever anti-extension, bigger guys should progress from knees-down rollouts, ditch the ab wheel, and do hand walkouts. This allows for more control, a wider hand base, and an easier way to keep the spine neutral while bracing the core. Hold each position for a 2-3 second count at full extension.

Related:  Core Training for Big, Heavy Dudes

Related:  Big, Thick, Chunky Ab Training