Because is more easy and gives more play showing domination the comparison between a big dick and a small one, it has the contrast of how boring is the sex for the female character with the small one vs struggling to satisfy the bigger one.
for example in the hand job is hard to do a length animation with a small dick, the movements would be very small to be noticeable or appealing in a animation
with smaller dicks is hard to animate short movement and looks liker her is not giving any effort to please her couple an gives the image
instead with bigger penis the woman looks more engaged in the act and show that she is giving effort to please her partner, showing that she is more submissive to the bigger dick partner than the small one even you can make this more evident showing that her have to use both hands, and is easier to animate or draw.
Another example would be the blowjob
with the small penis the woman has to do it soft to not slip out making look her likes she is not engaged in the act and that she is not submitted to her partner
Then comparing with the bigger penis she looks complete engaged and giving all to please her partner even if she is uncomfortable or tired by the size giving this image of submission to her partner, and is more easy to draw/animate, looks better for the most consumers of the game.
then is more hard to make appealing an adult game were a small penis is gonna steal the girlfriend to a someone with more bigger dick at least sexually, taking account the smaller penis has a limited set of sex scenes and the animation/draw are more harder to do well to convey passionate sex, slow scenes would be more for romantic/platonic setting not ntr setting.
even is more easy to sell to audience a nice guy with a bigger dick than one with small one, for example in sex if someone with big dick is thrusting slow is easy to say that he cares to not hurt her female partner, meanwhile the other way is more easy to say that his trying to not slip out his small penis to continue the sex showing incapability and conveying pity in her female partner.