The Islamic division of sins into grave and lesser is not a later scholastic overlay; it is read directly out of the Quran, and two verses carry most of the weight. The thirty-first verse of the fourth chapter undertakes that if a person avoids the grave sins they have been forbidden, God will remit their lesser deeds and admit them honourably. The thirty-second verse of the fifty-third chapter describes those who avoid grave sins and gross indecencies, excepting slight lapses. Between them the two categories are established, and the vocabulary of kaba'ir and sagha'ir follows.
The practical consequence is a scheme of ordinary maintenance. The lesser sins are held to be worn away by the routine of religious life rather than by any special act: the five daily prayers expiate what falls between them, one Friday assembly does the same for the week, one Ramadan for the year, and the reports that say so attach a standing condition, that the grave sins are being avoided. Piety here has an accumulating, cleansing function, and a person who keeps the round is not carrying the small stuff forward.
A grave sin breaks that arrangement. It is not covered by the daily round, and it requires the deliberate turning that the atlas treats under Tawbah: ceasing, regret, resolve, and where another human being has been wronged, restitution or that person's pardon, which the jurists hold God does not waive on the wronged party's behalf. So the two grades are distinguished less by how bad they are than by what it takes to be rid of them, which is a different axis from the one the Latin tradition uses.