What prevents insects from growing to enormous size is not a genetic firewall, but simple biological limitations due to their structure. An exoskeleton is a very heavy way to support an organism. The larger it gets, the thicker and tougher that skeleton has to be — until it’s so heavy the animal couldn’t move it. That’s limitation one.
The second limitation is their respiratory system. Insects have very primitive oxygen exchange mechanisms. The Earth’s atmosphere doesn’t contain that much oxygen. The larger insects get, the more oxygen they need, but their respiratory system can’t extract it. So that’s that.
When Earth’s oxygen levels were higher, insects did grow much larger than they do today. But only relatively speaking. The largest insects to ever live (that we’ve found so far) were dragonflies. But their wingspans were only 2.5 feet across. It’s big… but it’s not huge.
The largest land arthropod was a giant millipede named arthropleura. It grew to nearly 8 feet in length, and 2.6 feet across, but it was very flat.
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