There are still people out there who don't appreciate the fact that lifting an absolute weight for a finite number of reps won't have the same effect on two people if one is very short with small leverages and the other is very tall with large leverages. This transcends "training studies" and enters the realm of basic, 8th grade physics. The further away a load is from its fulcrum, the more demanding the load is on the fulcrum.
In the case of core training, that can mean trouble for big guys, very muscular guys, or guys with very long arms and legs. After all, there's a reason why those guys you see doing "bodyweight park workouts" on YouTube are the size of gymnasts and aren't allowed to ride some of the rides at the state fair.
Moreover, many indicators of true core strength don't take size into consideration at all, and your core may be considered "weak" because you can't do 20 leg raises or a full ab wheel rollout from the toes. Never mind that you're a lean 265 pounds and have excellent numbers in your big lifts.
What happens is that the vast majority of big or tall guys struggle to achieve very difficult positions and almost always end up tweaking their lower backs. It makes more sense to choose big-guy specific exercises, or to use smart modifications to existing core-exercise staples.
Lots of people can do ab wheel rollouts from the knees... and this would be a copout article if I implied tall guys can't get good at them. The problem comes when you lengthen the lever arm and do rollouts from the toes.
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.
I can almost guarantee that many big guys may be strong enough to muscle out a couple of decent-looking dragon flags, but it often comes at the expense of imposing a lot of pressure on their lower backs.
The best way to approach this is by simply moving some of the lower body mass closer to the fulcrum, thereby creating a friendlier force angle and cleaning up technique. You can do this by bending one knee to 90 degrees. It softens the blow this exercise places on the lumbar spine. Switch legs on each set.
A lot of big guys think they've "graduated" from bodyweight training, but they should consider this: All that extra muscle and mass they've added from successful training and eating makes bodyweight training even more effective.
When big guys do bodyweight training, they'll be moving around a heavier body, with heavier extremities, while helping to kick in conditioning. Case in point, dead bug variations can make the abs work overtime and can even serve as an indicator to true core weakness, if it exists.
Another benefit is that they'll be applying isometric pressure against something immovable, like the wall in the video. That means there's no measurable ceiling on how "strong" they can get at this movement, as long as they're putting out maximal effort each time. To progress this movement, try pulling and pushing immovable objects from different angles (overhead, sideways).
The biggest mistake people make when it comes to leg raises is initiating the movement with their hips. Big guys, with their long, heavy legs, usually make it worse since the hip junction is the only pivot point. To properly hit the abs, focus on rounding the spine while bending the knees.
If you struggle with leg raises from a dead arm position, or the overall loading is just too much to handle, there's no shame in forfeiting some of that load by resting the upper arms in sleeves.
This allows you to focus more on trunk flexion and having eccentric control through the negative half of the lift.
This exercise offers abdominal and oblique stimulation while also adding a hip mobility element into the mix. This helps the body distinguish the difference between the hips, pelvis, and lumbar spine so that they don't all move together as one unit.
Try elevating the feet. It increases the difficulty of the plank while also working on hip mobility.
All of these tips are fine and dandy, but they won't completely compensate for fat-assedness. Shedding excess body fat can help your core training while reducing residual low back pain and stress (especially if you tend to store fat in your stomach region).
Of course, this article isn't about how big guys can make their abs look better. Most of that comes down to a nice, clean diet that complements solid training strategies.