A few adaptations.
1: No heavy exoskeleton. The old square-cube law you know. Exoskeletons get heavy. These big ones have just enough to hold themselves in - a minimal flexible cuticle like that of a maggot.
- Apneustic respirations. Oxygen dissolves directly through the skin. Some maggots can do this, especially water dwelling ones. Amphibians and sea snakes can do this. The thin cuticle on your big bug facilitates this. It would be well suited for an area with minimal gaseous atmosphere.
3: Huge surface to volume area. This facilitates direct diffusion of oxygen to needy tissues. The big bug is flat. It might be like a big flat pancake.
Now we have a flat, pancake-like maggot. It is too big for legs, so moves by undulation - terrestrial flatworms and nemerteans do that, so no great stretch.
A creature like this is converging on a slime mold except it cannot flow and so must crawl. It could be very, very big. Such a large creature would also be fairly defenseless and so would need to either be unpalatable or to reside in an environment without predators. Maybe the deep earth, where it would itself be the apex predator. I bet they are down there.