@ZackB Thanks! I can only compare this jacket to IH type III jackets since I have five of them - each either made of a different fabric or a different colour.
Generally, I'd describe the fit of the Double Helix type III jacket as "boxier" in comparison. For example, the arms are noticeably looser than the arms of IH type III jackets, and they also don't taper as aggressively as IH Type III jackets do - especially towards the sleeve opening. The rest of the jacket fits almost exactly like an IH type III jacket would fit me in the same tag size - perhaps the front length is ever so slightly shorter (ca. 1-1.5cm) on the Double Helix, and the naval area isn't as much tapered as IH type III jackets tend to be.
Overall, if I had to pick an IH type III jacket that the Double Helix jacket fits closest to, I'd say the IHJ-69 which I have in brown (discontinued) and khaki. In terms of style, I could have went for the IHJ-49-BRN instead of this, but the Double Helix is, with its 0.8mm thickness, significantly (!) lighter and thinner; is more pliable, has a way nicer brown tone and it's also made of an insanely soft horsehide which almost even feels like sheepskin vs. the steerhide on the IH version.
Fun fact: I actually went for this jacket, because it reminds me of my IHJ-69 in brown (although the Double Helix comes in a way deeper and richer brown tone) which apparently my girlfriend decided to gift herself. So the Double Helix jacket will now be mine, and my brown IHJ-69 will be hers.