She doesn't seem to mind her weight gain as I'm sure she still has much of her speed. The calendar and clock reference her high speed. Her card's info says:
JIOJ-EN012:
My Plump Pony Pal Fili-Second
LIGHT / Plump Pony Pal-Type Synchro / Level 8 / ATK 0 / DEF 2750
1 Tuner + 1 or more non-Tuner "My Plump Pony Pal" monsters
Once per turn: You can switch this face-up card's ATK and DEF, until the end of this turn, and if you do, target 1 monster on the field whose ATK is less than this face-up card's new ATK; destroy that target.
Super Rare