It was way more. I agree with you. I'm making an observation that the spending on the yes side did not automatically result in a yes victory.
With respect to having the election and the referendum at the same time, there were a couple of things you mentioned. You get a higher participation rate when you have it with an election. You would get universal agreement on that. There is plenty evidence to demonstrate that when a referendum is held as a stand-alone you get a lower participation rate.
Is there any scholarship suggesting that you get a difference in the number of yes responses versus no responses? A referendum held at the time of an election produces a higher number of participants, but it also produces a large number of spoiled ballots. People have presumably said they were not sure they could make an intelligent decision. You might also get people who say they are voting no and generate more no votes than yes votes. Or is that not the case?