First, I'm talking about mass capacity, that is the weight a pawn can carry when caravaning/in their inventory, not carrying capacity (how many items they can carry when hauling).
How would one edit this value for a specific animal? The workaround-ish method would be to adjust their body size and leather/meat amount. So e.g. for Muffalo I gave it 5.0 body size and reduced leather/meat to 12/38 - this way they give the same leather/meat as before, but can carry a lot more in caravans. It's 35kg*5=175kg.
But can't I edit the mass capacity directly?