Step 1: Recall why dimorphism evolves at all.
Sexual size dimorphism, a size gap between males and females of the same species, evolves whenever body size affects the reproductive success of one sex more than the other. Whichever sex gains more from being bigger will, over many generations, become the larger sex.
Step 2: State what male-biased requires.
Since males are the larger sex in this case, the correct hypothesis must give males a reproductive or competitive reason to be big, one that does not apply equally, or apply at all, to females.
Step 3: Test the option that fits.
If large body size helps males win contests against rival males, defend territory, or be chosen by females, but gives females no comparable benefit, males keep getting pushed toward larger sizes while females do not follow. Over time this produces exactly the male-biased pattern in the question.
Step 4: Test the options that predict the wrong sex getting bigger.
Giving the competitive advantage to females instead of males would make females the larger sex, the reverse of what is observed. Likewise, a positive link between clutch size and female body size rewards bigger females with more offspring, again pushing females to be the larger sex, not males.
Step 5: Test the option with no size effect at all.
If mating success for both sexes is unrelated to body size, there is no selection pressure on size in either direction, so no dimorphism should evolve in the first place, male-biased or otherwise.
Step 6: Final Answer.
Only the hypothesis that size helps males compete, without a matching benefit for females, predicts males becoming the larger sex.
\[ \boxed{\text{Body size gives a competitive edge to males but not females}} \]