HMMM.
Emailed Freya Lingerie asking about the restricted sizes on their new bra lines. They offer up to K and J cup in their full cup, plunge and balcony bras, but their longline bras in the exact same ranges stop at a G cup. The reply I got was that they felt the bras did not offer enough support above a G cup.

I’m wondering how badly you have to design a longline bra - something with a wider band than a standard bra - so that it is LESS supportive than a normal one with a more narrow band.

I mean… what did they DO to the longline (which should really just be a standard plunge, balcony or full cup with an extra wide band around the chest) to make it less supportive? The bra still has underwires, and wide shoulder straps, and fabric covering the cups with shaping in the way the fabric is cut. The only difference should be that the band extends an inch or two below the bra, resulting in it having a wider piece of elastic and more hooks at the back, things which in normal bras mean even better support.
Does anyone have any clue?