It's possible the ice was made by a young, and therefore smaller, Bewilderbeast. We know that the fully grown ones can create ENORMOUS structures in one blast, so that one could feasibly have been made by one maybe Hookfang's size.
I suppose some irl friends and I who’ve chatted about this always presumed the Bewilderbeast we may find in RTTE S6 would be a juvenile, too. It’s cool to see someone thinking along the same lines. I still say size of Bewilderbeast in a cave tunnel, regardless of its age, is technically somewhat problematic because Drago’s Bewilderbeast is very young but already quite large. Bewilderbeast grow up fast. I’d probably say that this dragon is larger than an adult Monstrous Nightmare, but yes, certainly smaller than the Bewilderbeasts of HTTYD 2 by a lot. So if we had a young one that could fit in those tunnels, it’d almost have to basically be a baby… which leads to the question of how a baby got down there.
None of those questions are irreconcilable, though. I mean problematic to mean that it’s something that has to be considered with qualifications, not something that is impossible to have happen or explain. The cave tunnels we’ve seen beneath Berserker Island haven’t been that big, but that doesn’t mean all the cave tunnels are that narrow. There could be much wider tunnels that would make a Bewilderbeast dragon of any age more likely to dwell in them. As far as age, a hatchling would not hatch down there because an adult could not lay eggs down there (and the likelihood of the egg rolling in there seems extremely small). However, not all creatures in the animal kingdom have attentive parents raising them. And there’s always the young Bewilderbeast getting lost besides. It’s not likely but it’s not impossible either. It can be done.
Story-wise I also would suspect a juvenile makes more sense. Story would flow better. There’s not as much that can be done with the villains interacting with a full-sized dragon. More can be done with a young one. So I suspect that the villains almost have to be tackling a young one. Guess we’ll end up seeing! Should be interesting!