(not native, but I guess)
cea mai mare = the biggest
cea mai frumoase = the most beautiful
but maybe cea mare = so big, this big, uniquely big
So it is not a region that happens to be big among other things (regiunea mare și uscată) but the region known for being the big one like the "The Great Region" (regiunea cea mare).
A bit like Alexander the Great...?
Romanian natives, please correct me!
because the immediate descriptor of the region is "cea mare" which is big and the secondary is "este frumoasa" which is "is beautiful". It's like the subtle difference between "the tall man is ugly" and "the ugly man is tall". You're using the first adjective as the identifier of the subject, and the second adjective is the purpose of the sentence