"The Mountain" is looking more like a big hill now ... 'cause he's lost a TON of weight over the past two years -- and check out his new bod, he's SHREDDED!!!
Hafthor Bjornsson -- who starred as the big beast on "Game Of Thrones" -- has transformed his body since retiring from strongman competition in 2019 ... moving to the boxing world, and the results are impressive as hell.
Just two days out from his big celeb tilt with arm wrestling legend Devon Larratt, Bjornsson -- a 3x Arnold Classic and 2018 World's Strongest Man winner -- showed off all the progress he's made with his physique, revealing chiseled abs, pecs and arms.