The eight-bar blues is two lines of four, and what marks it out is how early it reaches the dominant: in the second bar, where the twelve-bar waits until the ninth. The shape declared here is the whole-bar form printed for Key to the Highway and J. B. Lenoir's Slow Down: home, dominant, two bars of the subdominant, then home and dominant twice over. The eight-bar has more variants in circulation than the twelve, and the recording of Key to the Highway itself splits its last two bars between two chords each; this page reads the whole-bar version as printed rather than averaging the variants into a shape nobody plays.
Reading it bar by bar
Three of the eight bars are dominant bars, more than in any other form on this site, and the dominant is where the blues scale rubs hardest. Over it the root of the key sits a semitone above the chord's third, the flat third a semitone above its fifth, the flat fifth a semitone under its root, and the flat seventh of the key becomes the chord's own blue third. Only the fourth and the fifth of the key are chord tones there. So an eight-bar spends a larger share of its length rubbing than the twelve-bar does, and the pair of subdominant bars in the middle is the one stretch where the flat third belongs to the chord, and the only place the fifth of the key turns into colour, as the ninth.
What a short form does to the feel
With the dominant arriving in the second bar, the form never settles into the long stretch on the home chord that the twelve-bar opens with. The pull away and back comes round twice in eight bars, and the lines are short enough that an answer follows almost as soon as the phrase is out. Compare the tables: the same six notes against the same three chords, packed closer together, so the rubs come thicker and the release is shorter.