Kusari can be dissapointing, I know
The cruise flame matches proportions, the normal impulse speed flame on the A-J Kusarian engine series is supposed to be that way, very small. That's how it is on a lot of ships that can change engine.
Not a bug, can be very annoying for someone who wants his/her ship to look good, but not a bug.
If there's any solution I can give you, is to, well, buy another engine, if you want a bigger flame.
i reccomend rhein VP series (Not only theyre rheinic, but VP series always had big flame)
Keep in mind I am no dev, but as far as I know, this is how it's supposed to be.