:GetExtentsSize() Not working?

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1

I made a small script to automatically weld and assign hitboxes to a pet pack i got. the results were not as expected…
The script:

local PetPack = game.Workspace.PetPack
for i, v in pairs(PetPack:GetChildren()) do
    local hitbox = Instance.new("Part", v)
    hitbox.Name = "Hitbox"
    hitbox.Transparency = 1
    hitbox.Size = v:GetExtentsSize()
    hitbox.CastShadow = false
    hitbox.CFrame = v:GetModelCFrame()
    for i2, v2 in pairs(v:GetChildren()) do
        if v2 ~= hitbox and v2:IsA("BasePart") then
            local weld = Instance.new("WeldConstraint", hitbox)
            weld.Part0 = hitbox
            weld.Part1 = v2
        end
    end
    v.PrimaryPart = hitbox
end

(the objects in the pet pack folder are all models)
I got this as a result:

image
image1090×603 123 KB

Whats wrong here? I already tested anchoring it and reexecuting the script, same result.
Pet pack that was used: Pet Pack - Roblox