What was Moshe's question?
Rashi: He asked Aharon to what extent they forced his hand, that caused him to bring this sin upon them. 1
Ramban: He was asking Aharon (rhetorically) what the people had done to him, that caused him to react in a way that brought about their destruction. 2
Seforno: He was actuelly taking Aharon to task, not so much for the sin of the Golden Calf, but for the elation that accompanied it, 3 which was worse than the sin itself - and it was for that, that Moshe held Aharon responsible, by announcing "Chag la'Hashem Machar". And this is why he added the word "Chata'ah Gedolah" here. 4
Vayikra Rabah (7:1): Aharon took a hammer and bashed the Eigel, to show that it has no power. Moshe criticized him, for it would have been better that they be Shogeg (if they thought that it has power) than Mezid. 5
The Ramban queries this explanation, based on the fact that Avodah-Zarah, in all its forms falls under the category of 'Yehareg ve'Al Ya'avor'. And he suggests that perhaps Moshe said this in order to bring out Aharon's guilt. See also Sifsei Chachamim.
In the way that David spoke to Yonasan. See Shmuel 1, 20:1 (Ramban).
And in his Tefilah later, in Pasuk 31, as well the word "Chata'ah" that Hashem added in the thirteen Midos (34:7), and "Chatoseinu", that Moshe added in his Tefilah there, Pasuk 9 (Seforno).
Ginas Veradim (OC 1:12): Yisrael already knew that idolatry has no power, and they only served the Eigel in order to allow themselves Arayos. And Hashem was angry with Aharon, beacause through him people would know that Yisrael were Meizid.
Why did Moshe refer to the sin as "Chata'ah Gedolah"?
Refer to 32:21:1:3;
Vayikra Rabah (7:1): Aharon took a hammer and beat the Eigel with it, to demonstrate that it has no power. And Moshe referred to it as Chata'ah Gedolah, because it would have been better for them to have been Shogeg (had they thought that it has power) than Meizid. 1
Ginas Veradim (OC 1:12): Yisrael already knew that idolatry has no power, and they only served the Eigel in order to allow themselves Arayos. And Hashem was angry with Aharon, because through him people would now know that Yisrael were Meizid.
Why did Moshe refer to the sin of the people ("Ki Heiveisa alav Chata'ah Gedolah"), and not directly to the sin of Aharon?
Ramban #1: He avoided attributing the sin to Aharon directly out of respect for his older brother.
Ramban #2: He did so because in his heart, he knew that Aharon was innocent, inasmuch as he had not the least intention of sinning - and he therefore placed the blame on him, not for actually sinning, but for causing the people to sin by failing to rebuke them. 1
