Since you guys on Patreon may not know this, Xion was referring to Namine cracking a small boob joke about her. Xion's kinda insecure about being both short and having a small chest, so she hates when people remark on either of them.
This was the first time any of the girls had a serious falling out with each other. In the end, I'm glad they made up though! Uncensored in the attachments!