I feel like we won't see an adult Varshahn, because the fact Varshahn was a child was probably reflective of the fact that, as far as the First Brood ggoes, Vrtra is the baby of the group. He presents as a child because by his timescale, he is one.

Also it probably has a practical element that, since that form was made in part to travel through that voidgate, it makes sense that it's smaller.

And also that Varshahn is a pretty nice unique model. Why change that up when you could get your money's worth out of it?