Even with a spherical sub the diameter impacts a lot of things. For example a large sphere sees significantly lower pressure across the side facing the surface than the side facing the sea floor.
At the depth of the titanic to see a 1% variation in pressure between the top most and bottom most points of a sphere, the sphere would need to be 40 meters in diameter. For context, the pressure vessels of the largest submarine in the world have a diameter of 10.9 meters. Note that pressure at a given depth varies due to things like temperature fluctuations, ocean currents, and even variation in Earth's gravity. Further, the walls of pressure vessels distribute the load - any variations of the pressure get averaged out. It's the same principle as a dome - every element of the sphere is pushing against the adjacent elements and resisting being pushed by those adjacent elements. At the size scale where this is no longer the case, you're not building a pressure vessel. If you're making a dam or a hollow column going down into water, or perhaps a massive dome on the ocean floor, you would need different equations. Even for a submarine you may be concerned with things besides pressure resistance, like collision or sea keeping, as previously stated. But from a pressure resistance standpoint the diameter to wall thickness requirement holds equally true for small exploratory subs and the largest military subs.
A sphere is a great shape for dealing with such forces but it’s just a more complicated system. Rotation can cause metal fatigue, openings get more complicated, etc.
> For context, the pressure vessels of the largest submarine in the world have a diameter of 10.9 meters.
Few subs can reach the titanic at 12,500 feet, at more common crush depths and especially non spherical geometries it’s very much worth considering. Subs often dive and surface at a significant angle.