there are circumstances where the smaller less athletic fighter pretty much always wins. when it comes to striking, short guys with long arms are more dangerous than tall guys because the only big advantage to height is having more reach on average.
and, low weight class male trained fighter pretty much always beats untrained bodybuilder as long as he's decent. all the counter examples people quote me with when i say this are boxing matches where the bodybuilder trained for 6+ months and 6 months of boxing is a GULCH of skill gap over untrained. and they always cherry pick shit short fighters at the end of their careers.