That look she gives him. Just acceptance of her being "fat".
I also notice a change in Spike's behavior. When Twi was more confident he was fine joking and jabbing at her. Now that she seems to be feeling low he feels bad when he accidentally mentions her weight.
He really does care about Twilight.
His character is evolving. We are watching her go on a downward spiral.
That expression by Twilight is classic.