What is the ruling on a worshiper deliberately rising from prostration out of fear of his 'awrah being exposed due to a torn garment, and is this considered hidden shirk or a major sin?

If the worshiper performs the minimum sufficient prostration, there is no harm in shortening it to avoid hardship, and this is not considered polytheism, as shortening the prayer is sometimes legislated to alleviate difficulty. This is indicated by the Prophet’s (peace and blessings be upon him) hadith: "Indeed, I stand for prayer intending to prolong it, then I hear the crying of a child, so I shorten it, disliking to burden his mother." As for wearing tight trousers, it is disliked even outside of prayer, because it defines the ‘awrah (private parts), as stated in Al-Sharh al-Kabeer by Al-Dardir.