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Blueface was skinnier than a pole when he was on the outside ... but now that he's on the inside -- that is, in state prison -- he's become totally buff with a buncha new tats!

The "Thotiana" rapper posted pics of himself standing with a couple of his boys inside California State Prison, Los Angeles County -- and they show Blueface has beefed up BIG TIME! You might even have to do a double-take because of his physical transformation.

Plus, his face is now completely tatted up, which is a drastic escalation from all the ink on his body before he landed in the California penal system.

In one prison photo, Blueface stares emotionless in the camera lens with a caption that reads, "This is not a 50/50 yard."

It's not clear what exactly Blueface means by that. But, you may

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