Everyone says Quantitative Easing and changing the treasury maturity structure are equivalent (e.g. Paul Krugman, James Hamilton, Robert Waldmann, Robert Hall, Robert Barro).
However, if you look at each bond market through surplus analysis, you can see the difference between the two operations.
A. Changing the treasury maturity structure
A-1. Long-Term Bond Market
When the government reduces the issuance of long-term treasury bond, the supply curve shifts from "Supply Curve 1" to "Supply Curve 2". As a result, total surplus declines (shaded area). The government surplus changes from OBCD to OB'C'D'. Bond buyers' surplus changes from DCE to D'C'E.
(Here I assumed that the government doesn't change the amount of issuance according to bid-price, so that the supply curve is vertical. The government may have the reserve price under which it cancels the issuance, but here I assume that the reserve price is zero for simplicity [Although this is unrealistic assumption, it doesn't affect the main observation of this post]. In that case, the government surplus equals the amount of issuance.)
Of the bond buyers' surplus decrease, DFC'D' transfers to the government. The other part of the decrease, FCC', is the deadweight loss.
A-2. Short-Term Bond Market
The government wants to offset the above reduction in the revenue it raises from the long-term bond market by increasing the short-term bond issuance. So, the supply curve shifts from "Supply Curve 1" to "Supply Curve 2" in the short-term bond market.
On the other hand, as short-term interest rate has hit the zero bound, the demand curve is flat. That is, T-bill and the money is equivalent in this market (That is the point many commentators - such as Krugman - emphasize). So, the bond buyers' surplus doesn't change before and after the issuance increase.
As a result, only the government's surplus changes to compensate the reduction it suffered in the long-term bond market (shaded area).
B. Quantitative Easing
Long-Term Bond Market
In Quantitative easing, the Central Bank participates in the purchase of the long-term bond. So the demand curve shifts from "Demand Curve 1" to "Demand Curve 2". In that case, the bond buyers' surplus doesn't change; it just shift from DCE to D'C''E' (*1). On the other hand, the government's surplus increases by the shaded area DCC''D', which equals ECC''E'.
That government's surplus increase is the amount of "seigniorage" which the Central Bank monetized. It shows up explicitly in the change in bond price as shown in the above figure, so its effect on inflation expectation is more "direct" than in changing the treasury maturity structure case.
(When the Central Bank doesn't purchase the bond directly from the government, some of the surplus is diverted to the arbitrage profit of the bank. But if the competition among banks works well, that arbitrage profit approaches zero in the limit.)
(*1) Of the bond buyers' surplus after the shift D'C''E', C'C''E'E is the surplus of the Central Bank. So the private buyers' surplus is D'C'E, which equals to the bond buyers' surplus when the bond supply was decreased (A-1). However, there is no deadweight loss this time.
In sum, changing the treasury maturity structure ends up in the decrease of the long-term bond buyers' surplus with deadweight loss. In contrast, QE increases the government's surplus while keeping the total bond buyers' surplus unchanged. (Although the private bond buyers' surplus decreases in the latter case as much as the former case, there is no deadweight loss.)
Some may say that decrease in the issuance of long-term treasury bonds, along with decrease in the surplus of their buyers, is not such a big deal. But didn't Ricardo Caballero and Brad Delong emphasize about the current shortage in safe assets? Decreasing the issuance of long-term treasury bonds exacerbates that shortage.
While the Central Bank's purchase of the bonds seems to be equivalent to the decrease in the issuance of the bonds on the consolidated government basis, total outstanding of the bonds is different in each case nonetheless. Thinner total outstanding makes the market more vulnerable to the demand shock, which is likely to occur considering the craving for safe assets Caballero and Delong noted.
Besides, that difference in total outstanding involves deadweight loss as shown above.