I really think that everyone may be missing the wood for the trees, here. It's not a question of whether the cardigan will fit. The armour is a metaphor for... hang on, let me take a few steps back.
Think back to the introduction of Bubbles and Corpse Witch. Bubbles in her fearsome, huge, war armour, with the surprising name Bubbles. We soon learn that Bubbles is sensitive and intelligent, but is unwilling to speak of her past, and is hiding her emotions and her vulnerability behind a metaphorical and literal suit of armour.
In contrast, Corpse Witch - who, in a humorous contrast with the title of the comic introducing Bubbles, was introduced emphatically as CORPSE WITCH, which everyone took IMO a little too literally - Corpse Witch is outwardly pretty and delicate, but is, as we have since learned, insensitive, manipulative, and perhaps a little hateful.
The counterpoint is interesting, and pretty clear.
Where am I going with this? We know that Bubbles is still not entirely willing to open up emotionally. For example, she blushes when she admits that she sews, which is a "soft" activity. A cardigan would similarly soften her outward appearance, and compromise the emotional armour she's built around herself. She's not quite ready to do that yet, even with Faye.
It has nothing to do with whether the cardigan will fit. When she states emphatically that she "WEARS HER ARMOUR" she is saying in no uncertain terms that she's not quite ready to drop her emotional guard.