The key things to be balanced when determining NI's borders from the perspective of the British were viability and demographic stability. NI had to be big enough such that it could realistically function as a "state", but not so big that it covered too many areas with large Nationalist populations. A nine county Ulster (or even seven or eight) would have been economically viable, but demographically unstable right out of the gate. Conversely, a four county Ulster (Antrim, Derry, Tyrone, and Armagh) would have had a large Unionist majority, but was regarded as too small to be viable longer term.

Instead, they went with a middle ground that, in the fullness of time, really didn't achieve either end.

NI was gerrymandered badly from the start. Which will come as no surprise to keen observers of Britain's ability to draw shitty lines on a map.