The two main factors would be:
- sexual selection;
- reproductive success non-dependent on physical strength and body size.
Sexual selection is natural selection based on preferences for sexual partners. It is speculated that sexual selection is the main reason for higher-pitched voice and sub-optimal fat distribution in human females.
Your hypothetical hominids need to favour traits that you listed as desirable and select for them. As time goes by those traits will become dominant in the general population.
Reproductive success refers to an individual's ability to produce offsprings that become part of the breeding population once they mature. If greater strength and/or bigger body size result in reproductive success your species will eventually evolve to be bigger and stronger. Sexual dimorphism will occur if requirements for reproductive success differ between sexes.
A specific method for exclusion of strength and body size from traits important for reproductive success will depend on your species physiology, habitat, and social structure. For example, hidden ovulation (like humans have) increases the necessity for the monopolisation of females by males. A habitat full of dangerous predators and sex roles where males are the main protectors will favour strength (for fight) or agility and stamina (for flight). Social structures with polygamous mating will most likely favour strength if violence is the main method of harem protection.
Another important aspect is male-male interactions. Species where male combat is common tend to have males stronger and bigger than females. If your male hominids do not use violence to win against other males differences in strength and body size will be less pronounced.