i had that problem with my baby banana bp, i had to put him in a 6qt tub because he didnt feel secure enough in a tub that was only twice as big and he was refusing meals.
also in my opinion the terrarium you are using might be a bit big for a baby but it does look pretty nice.
You can , I feel , make a Viv/rub appear smaller by adding , fake foliage, branches / bark pieces and more hides ... You don't have to actually go to a smaller container , in my experience anyways ..