Understanding scanline thickness

First of all, I´ll make the distinction of "color scanline" and "blank scanline" (before getting 15 replies explaining that scanline is actually the signal beam and not the black line lol).

Some CRTs show the color scanline with variable thickness, being really thin in dark colors and thicker on bright colors, to the point the color scanline touches the one above and bellow (making the blank scanline dessapear). I find this to be quite nice to look at, but no all my CRTs do it and I don´t know what is the differential factor in play here.

I´ve seen people calling this "bloom", but to me bloom is the geometry expansion during bright colors, not the thickness of the individual color scanline, this expansion can take place on the whole screen or part of it (a bit is ok, but some 80 crt go all over the place with this).

I think it is contrast related, but I can´t replicate it on all my crt, even with contrast all the way up (all the colors get bigger, not only the brightest ones). So it has to be something else.

do you know how to manipulate this variable thickness??