Generally GPU load scales with resolution and graphical fidelity, while CPU load mostly just scales with framerate irrespective of resolution or graphics settings - so you might be CPU bottlenecked at max settings 1080p with an average CPU and a mid-high end GPU, but even with the current highest end GPUs and a mid range CPU you're likely not bottlenecked by the CPU at 1440p or 4k because the GPU isn't pushing out as many frames.