6ix9ine got into the nitty-gritty of his recent weight gain/loss Wednesday on 'TMZ Live' -- where he dished on how exactly he ended up a full 70 pounds heavier within a few months of getting out of prison.

Ya might've heard ... Tekashi says he hit a whopping 204 lbs. during home confinement last year -- he was about 130 lbs. when he left the pen -- but miraculously dropped the poundage in record time ... something he alluded to recently on social media.

He explains he was able to get himself back down to around 140 lbs., adding this in celebration ... "In the chaos of everything that is going on in your life, tell yourself 'I control what happens in my life' do NOT let anyone tell you that things you want it’s not possible, that’s FALSE!"

So many questions here, which Harvey and Charles dug into with Tekashi -- such as, what the hell was he eating to gain so much weight -- and perhaps just as important, how'd he lose it all so quickly???

Tekashi's got the answers ... and his solution might be a bit more primitive than any fad diet out right now. Turns out, the answer to tons of Chinese food is ... sweat and misery.