They were pretty big though. This goes back to debate about cultural impact vs sales. It wasn't just the music. It was the fashion, unfortunately, the gang shyt, the lifestyle, etc...obviously they weren't as big as G-Unit on a mainstream level, not even close.
But nikkas was not dressing like Bank and Yayo. nikkas wasn't making music like them. Nobody wanted to be them nikkas. nikkas wanted to be 50 and 50 only. Conversely, nikkas was heavily influenced by all of the big 3 of Dip Set.
Damn near all the popular drill artists in Chicago from Fredo Santana (RIP said he calls himself Santana because of Juelz) to G Herbo to Lil Durk said they were all heavily influenced by Dip Set. The biggest rapper in the world at one point, Lil Wayne, was damn near the biggest Dipset stan in the world. Look at how Drake treats Dip Set.
Their influence culturally far outweighs their record sales.