Non-Sunni majority areas laid down their weapons soon after US invasion of Iraq, Sunni areas didn't do it because of majority of Sunni Army personal, hence most of them got bombarded and a lot got killed, their is a fear that deaths of Sunni's which were already above a million have spiked under Shia regime's offensives in different regions. I am not saying that Shia were not killed in US invasion or by ISIS. but right now their is a mass genocide of Sunni's going on and Shia are a part of that. If one thinks that "ISIS" will give up Musul easily, then they are living in a world of idiots. "ISIS" is not going out until majority of Muslims is turned into a minority. if you don't know, none of the Sunni soldier is sent to Musul.Iraq before the 2003 invasion wasn't a Shia majority country. A grand door was opened to Iran after US messed up good in Iraq. What Iran did was exporting Shia citizens in Iraq in order to manipulate the numbers to gain controle of the governship in Iraq - creating an oil rich puppet country.
And besides, even if the Sunnis are minority today, we are talking about a huge minority. I would guess around 35-40% atleast.
I identity myself as Muslim and not as Sunni.