If I were to graph myself speaking those two words I expect I would have two spikes on the k's of kiki, but bouba would be generally flatter. We probably have

If I were to graph myself speaking those two words I expect I would have two spikes on the k's of kiki, but bouba would be generally flatter. We probably have abstractions, connotations, that these sounds share with the physical ideas of sharpness and soft-formed-ness (what word am I looking for). Then, we likely choose our words against these abstract connotations. That's what I guess before I would look at your explanation as right.