I'm the biggest boss that you seen thus far
Rick Ross claims that he is the best out of everyone who has lived, so far.
Being a boss means having power. Being a big boss means having a lot of power. Being the biggest boss means that he has the most power, and being the biggest boss that anyone has seen thus far means that nobody has ever had more power than he currently has.
It may also be an allusion towards Rick Ross' size, since he is overweight, and therefore big. There have been numerous successful obese rappers, but Rick Ross just may be the most obese of them all.
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