What do we call Gunah Kabira (greater sins) is different from daily routine mistakes and Gunah. It caused severe affect in ones life and has severe punishment on the Day of Judgement. Allah say in Sura Nisa verse 31:
إِن تَجْتَنِبُوا كَبَائِرَ مَا تُنْهَوْنَ عَنْهُ نُكَفِّرْ عَنكُمْ سَيِّئَاتِكُمْ وَنُدْخِلْكُم مُّدْخَلًا كَرِيمًا
If you shun the great sins which you are forbidden, We will do away with your small sins and cause you to enter an honorable place of entering.(4:31)
But we should not consider any Gunah as it is small because it’s long term effect is that a person who considers any Gunah small becomes habitual of doing them. Whilst Gunah e Kabira is its a very serious and dangerous action according to Islam.