Yep it's a rather fancy effect, it should be scaled down a bit.
I sorta...can't do that.
So, here's the deal. Scaling down the particle radius makes the particles, visually, further apart relative to their size. We don't want them too far apart -- that looks weird; it makes it too obvious that this trail of rocket flames is actually a series of discrete round objects.
So there are two potential ways of dealing with this: spawn particles more frequently, or increase the particle speed so that they're more clumped together (then maybe increase the particle lifespan so that the trail is the same length). Unfortunately, I can't do either of these things.
I'm already spawning one set of particles every frame. AIUI, pspew can't do any more than that. I can increase the particle count, but for the Plume type -- which is what I'm using -- that just makes it so that more particles spawn adjacent to each other each frame. (Helix actually will do this properly, but, well, then it's a helix.)
Now, if I used $inflight effect in conjunction with -part.tbm, I could spawn arbitrary numbers of particles each frame and have it work correctly -- but -part.tbm has a huge limitation which makes it nonviable for this task: it can only spawn particles at the exact location of whatever's spawning them, not in a disk like $pspew's default or plume types. That means that if I want any irregularity in the particle positions, rather than having them be a perfectly straight line, I'd need to have them expand outward from the central line, and that wouldn't end up looking good at all.
So what about increasing the particle speed? That should work, right? There's a +vel option under $pspew. Well... I'm using plume, so that I can spawn the particles in a disk at the projectile and then have them move inward to create a nice rat-tail effect. Now here's the thing. For reasons that completely escape me, for plume and ring type effects, the +vel field...doesn't...****in'...work. It's a valid field! You can enter whatever number you want, and the game doesn't throw you an error or anything. It just...doesn't do anything. The particles will be stationary, no matter what number you enter.
So that's where we're at. I'll have more options if the +vel issue is...fixed? I assume it's a bug? or if -part.tbm's functionality is expanded to allow for disk or sphere spawning. I'll work on trying to submit bug reports/feature requests for those two issues -- for the second one, -part.tbm is already very very good, but improving it in that way would expand its usefulness dramatically, and I think it's very worth doing unless it would be super hard to implement for some reason.
Admittedly, I could just bite the bullet scale down the particles. Spawning them in clusters like I have it now does make it look a lot more natural even if they are far apart. I'll experiment with that.