The pro-UK Better Together campaign complained that legal spending limits were too low following last year's independence referendum.
The 'No' campaign told the Electoral Commission that £1.5 million was not enough to communicate effectively with all Scots.
In a submission to the regulator, Better Together put the cost of posting a letter to every registered voter in Scotland at more than £2m.
Because of the spending limit it had had to use a system of targeted campaign materials instead, it said.
The rival 'Yes' Campaign, Yes Scotland, did not comment on the spending limit.
The complaint is included in new report on the referendum by the commission, which also confirms that the 'No' side received almost 50 per cent more in donations than 'Yes' campaigners.
Total reported donations were more than £7 million, with those registered to campaign for a 'No' given £4,327,677 and for a 'Yes' £2,990,868.
That meant that the pro-Union side had 59 per cent of the reported donations, while independence campaigners had 41 per cent.
But the spending between two sides did not differ so wildly.
The report found that the gap was just £400,000.
Those registered to campaign on either side of the debate also reported spending more than 70 per cent of their total combined individual limits.
And both lead campaigns spent almost exactly the same amount - Better Together £1,422,602 and Yes Scotland £1,420,800.
The report says that political parties also reported spending "relatively similar" amounts - totalling around £1.3m.
Of the 42 campaigners registered were evenly split, with 21 supporting each side.
Registered campaigners reported spending a total of £6,664,980 in the independence referendum, while they received donations and loans totalling £7,318,545.
SNP MSP Christian Allard said: "The strength of the referendum wasn't the sums of cash spent, which seems to be No campaign's chief concern, but the sheer number of people who engaged in the political process, the excellent voter turnout - and how Scotland is now the most politically energised country in Europe."
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