Why not Tá?
Certain predicate adjectives (e.g. those of indefinite amounts) use is rather than bí.
What is an indefinite amount????
An amount that is not defined, like loads or many or lots. As opposed to a defined amount like 30 of them or 2.8 square metres or half a kilo.
See: "adjectives of equation (adjectives about an indefinite amount)" here: http://nualeargais.ie/gnag/kopul5.htm#besitz.
Would "is mór atá eatarthu" work here too or is that totally wrong?
Why not tá móran eatarthu? It accepted níl móran eatarthu for there is little between them so is this not the opposite?
I tried mórán too - it seemed a bit larger than 'mór'. However, Duo had other ideas!
Why not accept “ níos mo” thats in the hints
I think "níos mo" means "more"