Boxer Jarrell 'Big Baby' Miller

Heavyweight boxer Jarrell "Big Baby" Miller says the failed drug test that forced him out of his fight against Anthony Joshua cost him around $30 MILLION!!!!

... and yeah, he still thinks about it all the time!!

Remember, Miller was the guy ORIGINALLY slated to fight AJ on June 1 at Madison Square Garden -- but when "Big Baby" failed the test, Joshua's people signed Andy Ruiz as a last-minute replacement.

And, we all know what happened next -- Ruiz beat the dog snot out of Joshua ... and now Andy is trying to lock up a $50 million rematch deal.

So, when "Big Baby" joined the guys on the "TMZ Sports" TV show (airs weekdays on FS1) -- we asked if he ever thinks to himself, "That shoulda been me!"

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