Is sitting while performing supererogatory prayers (Nawafil) and night vigil prayers (Qiyam al-Layl) due to excess weight, even with the occasional ability to stand, considered a sin that leads to feelings of guilt?

There is no harm in performing the supererogatory (Sunnah (definition) and Nafilah) prayers while sitting, even if one is able to stand, except that the reward for the one who stands is double the reward of the one who sits, provided there is no excuse. This is based on the Prophet's (peace be upon him) saying: "The prayer of a man performed while sitting is half the prayer," and "If he prays standing, it is better. And whoever prays sitting, he receives half the reward of the one who prays standing. And whoever prays lying down, he receives half the reward of the one who prays sitting." As for the one who has an excuse, his sitting does not diminish his reward, based on the Prophet's (peace be upon him) saying: "If a servant falls ill or travels, what he used to do when he was a resident and healthy will be recorded for him."