As TheLBLMan posted, bucks put on a MASSIVE amount of muscle in October, leading up to the rut. Their testosterone levels are about 10 times what a teenage human male has in his blood. Hunters used to talk about how bucks' necks "swell" for the rut. Their necks don't swell; that's pure muscle. And post-rut, bucks can lose weight even faster due to exertions during the rut and lack of time to eat. A mature buck can lose 30% of his body weight in a month. That's a 200 lb mature buck just before the rut dropping to 140 lbs in a month to a month and a half.
An example: Below is a buck I had been after for a couple of years. He is 5 1/2 in the below picture. I would put his weight in this picture at somewhere around 210-220 lbs live weight. The picture was taken October 12th. When I finally killed that buck Dec. 31st, very much post-rut, he only weighed 158 lbs live. He hardly looked like the same buck body-wise.