I think they're rationalizing it by saying that's a juvenile sandworm. The larger one with the round mouth is fully grown.
They probably just wanted a design difference between a smaller human scale threat and the massive ones that eat entire structures like spice harvesters.
I feel the same way. MMOs is just way too time consuming. And I feel like there is always some pressure to keep up. I already got that covered by work. Give me a small scale coop I can play with a couple of friends.
Funcom have a habit of having good ideas and not following through on them. The Secret World is the first time I encountered them. And to be fair the game has a story that I find really intriguing. It is a shame such a fun story is locked behind an mmo when as a single player experience it could be so, so much more.