They didn’t start off being the same, but they became the same.
Great Natural Bear was introduced as a minor character in the Bears’ Almanac in 1973 to show a bear hibernating and doing other “natural” bear behaviors.
Bigpaw was introduced later as the main character in the Thanksgiving TV special, The Berenstain Bears Meet Bigpaw, and he began to appear in other books, especially the chapter books, as an ongoing character. Bigpaw looked and acted just like Great Natural Bear. Though it was never actually stated, it was assumed that Great Natural Bear was the same character as Bigpaw but with a different name. Great Natural Bear can be viewed as a “precursor” to Bigpaw.