إِذَا صَلَّى أَحَدُكُمْ لِلنَّاسِ فَلْيُخَفِّفْ، فَإِنَّ فِيهِمُ الضَّعِيفَ وَالسَّقِيمَ وَالْكَبِيرَ
Meaning: When one of you leads people in prayer, let him lighten it, for among them are the weak, the sick and the elderly.
The imam considers the congregation’s capacity, whereas an individual praying alone has greater latitude for permissible length.