Don't mean to disagree with previous statements. I fully understand that "tightness" can be a very subjective thing to discuss. And I understand "tightness" most certainly can be related to the "feel" of a pickup.
But the way *I* have always described "tightness" in a pickup as "objectically" as I can (and I say "objectically" because I admit there is always room for interpretation and/or debate) is the same way you get an idea of how an amp's "tightness" works. If an amp has some low-end filtering going on at the input stage, pre-distortion, the amp reacts faster to palm-mutes and low notes, thus "feeling tighter". An unboosted Recto, for example, has very little filtering in the input stage compared to a 5150III. It doesn't help that a Recto's preamp is voiced super bassy either, but you can also get a 5150III to sound pretty much as big as a Recto in the bass with the Depth control, yet it will always come off as tighter because of the filtering that's going on pre-distortion.
I personally judge a pickup's tightness the same way. A pickup's EQ curve is always pre-distortion.
To me, while a JB is a fat-soundingn pickup, especially in the mids, its low-end is not really deep or resonant. A JB to me can be stuffy-sounding in the low-mids. Especially if you're used to actives. I guess that's what some people describe as "loose". But to me, a JB is not a bassy pickup. Hence, I don't find it particularly loose, personally. Stuffy, yes. Loose... nah. Not to me.
That's exactly why I cannot understand how people would feel the Custom 5, the Blackout AHB-1, or the X2N are tight pickups, when those are the bassiest, boomiest pickups I've tried. And to me, no passive pickup can ever be as tight as an EMG 81 because passive pickups simply do not have the HUGE low-end rolloff of a traditional EMG pickup.
JMO.