Compared to Wagner's massive operas, Mozart's longest operas seem light.
Next toWagner's massive operas, Mozart's longest operas seem light.
Compared to Wagner's massive operas, Mozart's longest operas seem light.
Next to Wagner's massive operas, Mozart's longest operas seem light.
The structure "next to" used for comparisons is very informal and wouldn't be correct on the GMAT. We must use the more formal, correct idiom: "compared to A, B."