Most ppl said that size,strength doesnt matters in fight..but why is there weight category in MMA competition?

most ppl think that small guys can take on bigger boys with their speed?how true is that?any videos links to prove it?
Update: well if both fighters are different weight classes but still have the mass,big muscles or even big bone structure..its still about person like bruce lee(skinny/underweight) or jet li (small frame)how do they survive in real fight with eg,couture,tito,brock lesnar etc?
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