Or HaChaim on Genesis 18:20/אור החיים על בראשית י״ח:כ׳

1 ויאמר ה׳ זעקת פדום ועמורה, כי רבה G'd said: "the outcry about Sodom and Gomorrha is indeed great, etc.;" the reason for the word כי, as well as the additional word חטאתם, plus the description כבדה מאד, require analysis. We must remember that in that era all the nations angered G'd by their conduct all the time. The wickedness of the Sodomites, however, was in a class by itself. This is indicated by the words כי רבה, "for it is great;" this explains why G'd seemed to mind the conduct of those cities more than that of any others. The additional word וחטאתם alludes to the fact that their wickedness included not only the metaphysical, i.e. idolatry, but also moral-ethical wickedness in their relations with fellow human beings. Sanhedrin 109 lists examples of the latter, describing how they tortured a young girl to death for having given bread to a stranger, something illegal in that town. The Torah itself alludes to their attempts to have homosexual relations with the angels who visited Lot in the guise of human beings.

2G'd also wanted to cool Abraham's enthusiasm for saving such a wicked people. This is why He told him that their sins were both numerous and of a severe nature. Originally their sins were "merely" numerous i.e. רבה; meanwhile they had become "severe," כבדה, also. When Abraham rescued the Sodomites from the four kings their sins had not yet been that severe.