According to a fatwa by a Egyptian Mufti, if an individual went on Haj and gained more weight in comparison to their state prior to going to Haj, s/he 's done a Haraam action.
According to a report by the news agency of Ahlul-Bait, Abna, this fatwa is issued regardless of the fact that pilgrims to Mecca and Medina usually do not do their daily activities and instead take time to pray, rest and visit holy places. Then it seems natural that they have less movement rather than what they have in a typical day and are rather prone to gain weight.
Doctors are of the idea that pilgrims should get proper nutrition and ordered sleep when they are in Saudi Arabia, because their body weakness would result to illness.