Its a very difficult thing to comment on, i'm fully aware as i'm sure some people will state, that depending on the height and temperature of the air that the plane is flying at, factors on wether it leaves such long contrails.
But from experience i often see planes flying at seemingly similar levels, with one causing a massive contrail that often turns into clouds and yet
the other just leaving a small one that slowly disapates behind it. (also to add i'm quite good at judging the type of aircraft so am quite sure its
often the exact same plane having these opposing results.)
Sorry i can't add any more on the matter........its just one of those subjects you have to be quite conservative on these days for fear of being