First of all...I don't think SD sold all that more than HHID at all. SD was a double album so the sells counted double.
Secondly EVERYONE's sells tend to go down after being in the game a long time. HHID was over 10 years later than IWW and 12 after Illmatic. Even huge artists from just a year earlier like Jeezy and Game who stayed on the same label couldn't sell close to what they did by 06. Pretty much TI was the only artist of that time who had an increase in sales. DMX also went to Sony around this time and couldn't sell shyt. Nas going to Def Jam was the best thing he could do and it worked well.
1). SD was a double album but was NOT counted twice as it was about 5 minutes short of the official length necessary for soundscan to double the sales.
2). SD sold 250k its first week, debuting at number 5 and went on to be certified Platinum. HHID sold 355k its first week, debuting at number 1 and went onto be certified platinum, making it the last Nas album to date to go platinum.
3). HHID’s first week outsold Stillmatic’s (340k) Gods Son (156k) and SD. Untitled’s first week (187k) also outsold Gods Son’s (Gods Son had stronger legs and was certified platinum less than a month after its release)
So in essence, The Def Jam/Nas era was a resounding success. One platinum album and 2 Gold albums out of 4 is NOTHING to sneeze at. Nas’s popularity as an artist actually INCREASED slightly once he signed to Def Jam as he then became involved in more marketing and touring ventures. You also saw more artists and media personalities showing Nas more public respect as an artist during his Def Jam stint. He was named the greatest lyricist of all time by CNN. Was placed on the cover of The Source, XXL, Ebony and other magazines several times after being ignored for a fee years by those outlets, began his successful partnership with Hennessey, and of course started QB Ventures.
All of this kicked off with his Def Jam deal