Code:
hooksecurefunc(MultiBarRight, "SetScale", function(self, scale)
if scale < 1 then self:SetScale(1) end
end)
hooksecurefunc(MultiBarLeft, "SetScale", function(self, scale)
if scale < 1 then self:SetScale(1) end
end)
With the code, the multibars would only scale themselves by the value of your UIScale.