I fused a Gravebane Wight with a creature having Big Boned. According to the description, this creature should gain HP based on its own ATK/INT/DEF/SPE stats then set them to minimum in battle. However it actually gains HP based on every creatures in my team and set all of their stats to the minimum too.
One of my creatures without Big Boned in my team:
Now with Big Boned in my team:
My creature holding Big Boned: