I just checked and United is flying 3 types of jets ORD-CID:
ERJ-145, ERJ-170 and CRJ-700.
The last 2 are the larger ones. ERJ-145s have the 2/1 configuration and the others 2/2
I suspect the 50 passenger jets you were referring to are probably pre-merger smaller United CRJ200s or something. ERJ-170 and CRJ-700 are about as wide as those, but just longer and about as big as you get with regional jets now that practically nobody flies BA146s. ERJ-145s are even smaller.
Need a bit more info on those 90 seat planes, like what kind they were... If they were Fokker-28/70/100 or DC-9, those were mainline jets