J frames are easy to conceal / so are K frames, I will occasionally carry a model 19 or 66 in a 2 1/2" or a 4" barrel ...in a Kramer IWB holster with a forward FBI tilt...same holster for either gun ....( so no, a 2 1/2", 3" or a 4" ...will all fit into a holster designed for a 4" )....so they're all equally concealable - in that holster ). Round butt vs Square butt - might make a little difference....
my primary carry gun is a full sized 1911, 5" barrel in .45 acp ...and again in a good holster, with a good belt, its easy to conceal too...
but what are you comparing the J frames to ...???