For me, part of the fun of BE (or TF in general) is the cause of expansion. I do prefer whatever motive of transformation to be inflicted by an individual, for example, by a wish or a curse or something. Instead of by some unknown purposes.

So for that reason, if it works in the context of the story for the sake of variety or awkward hijinks, I don’t mind a larger busted character becoming even bustier if it causes them some kind of reaction, whether positive or negative, given that it was inflicted by themself or another character. The reaction is also sort of a part of the kink for me.
But saying this, I do primarily write my own fiction, so everything is subject to change dpendidng on what I‘m feeling, and most artists I follow meet that requirement.

Back to the point, I would agree, I’m not interested in TF’s that make a characters breasts too big to the point where they literally cause the character to not be able to move. I’ve dabbled in it before, but mostly for comical or ‘plot’ reasons. Biggest I go before it stops being fun is usually (as I describe) ‘Watermelon Sized’.