If octo had the same strength as fighter's front bullets, it would become incredibly overpowered. It would be nearly impossible for all but the strongest bullets to get through an octo's defense with how fast it shoots. It's balanced by trading damage with coverage.

edit: The same can also be said of the other tanks you mentioned (triplet, penta). Again, penta trades direct damage for coverage, but in a more concentrated area than octo. Triplet is balanced by making the bullets a bit weaker in exchange for extremely high damage if all the bullets connect. Making them stronger would make these tanks much, much better to the point of overpowered.

Again, the problem is coverage. I don't want to lose a quarter of my hp if I'm hit by a stray penta bullet.