my armchair psychoanalysis:
fat rick occupies a tier of low level midwit where he is too stupid to achieve or perform well at anything, but just smart enough to be self aware of these shortcomings. that awareness hurts his ego a lot, and all his posturing and grandiose bravado is an attempt by him to protect his ego by never admitting failure or being wrong, ever, no matter the cost.
this is in contrast to someone like russell greer, who is also too stupid to achieve anything, but also lacks any self awareness, so he continues bumbling through life as a completely oblivious retard, forever ignorant of his shortcomings.