And that’s what this metric is measuring, the model finding hate more easily with “fat people are terrible” than “normal weight people are terrible”
Hateful expression is more prominent when it’s socially acceptable, and in American society at large, there are socially acceptable targets for hate that do not align with the majority/minority group division.
Consider, for example, the prominent, public, long-standing (and for some reason, tolerated) racial animosity between Asian and Black Americans.
You say you don't know why it's tolerated, but that's my point exactly; it's tolerated because it's hate towards a minority group (regardless of the source). It would be substantially less tolerated if it were hate towards the majority group.
Consider how often you’ve read “kill all men” or “kill all terfs”, versus “kill all women” or “kill all transgender people”.
Furthermore, consider the stronger emotional reaction you likely had to reading the second two, as opposed to the first two.
By contrast, trans-identifying people are comparatively celebrated in our society.