6 days of creation

“Say: Is it that ye disbelieve in Him Who created the earth in two Days? And do ye join equals with Him? He is the Lord of (all) the Worlds. He made therein Rawasy (stabilizers) from above and bestowed blessings in it, and gave due measure therein the sustenance, in four Days, for those who question.
And then He established it until to the sky, and it had been (as) smoke: He said to it and to the earth: “Come ye together, willingly or unwillingly.” They said: “We do come (together), in willing obedience.”
“And so He completed them as seven skies in two Days, and He inspired to each sky its command. And We adorned the sky of the world with lamps, and protected it. Such is the Decree of (Him) the Exalted in Might, Full of Knowledge.” [Qu’ran Surat Fussilat (41):9-12]

Why was the period of creating the earth and its sustenance four days, while the creation of the skies was two days since the skies are greater?