> The 'b' sound is a plosive too, by the way. It's voiced, vs the voiceless 'k'.

Yeah this I know, by "bouba" being opposite I meant the waveform.

Really the phonetic terms here are not actually relevant and have betrayed me. It's more that the 'k' sound is "more [ex]plosive" in that it seems to build up more pressure and turbulence (probably as you say, because the consonant is in the rear, so pressure builds and bounces around in the mouth more, whereas the "b" just mostly blasts out the front more quickly). And the "k" surely has a more peaky / spikey waveform in the audible range.