Most Helpful Guys
As a gamedev I could give you a whole range of answers, but let me start with the most obvious one.
We like to see big boobs in all their jigglyness. There are entire physics engines build to simulate the most bouncy of bouncy boobs. We don't spend money on this without reason, people pay for it. If people pay for it, we make it.
Another aspect could be found in character design, big boobs influence your judgement on a person's character.
A lot of main characters used to be androgynous removing features such as voice and removing aspects such as boobs. Also the clothes would often be in natural colours. This was all to make people relate to these characters as much as possible.
Now with these big boobs you can say it's the extreme opposite. Giving her huge boobs makes you feel distant to her character and changes how you perceive her as a character. People generally won't relate with her or see her as anything but a lust object.
For example, take Kairi from Kingdom Hearts, obviously this was mostly targeted towards an Asian target demographic, but she is basically seen also girlfriend material from the get-go. Her features are modest and she's quite relatable. Her boobs are of "normal" size for this exact reason, it helps how you perceive the character.
Not many of the replies here are well informed. There are a lot that are kind of right but allow me to be pompous and presume I know better.
It's a lot of factors.
A) Sex sells to men and women. Big breasts are a very clear sexualisation of a female character. It draws people's attentions and appeals to both men and women regardless of sexual orientation. You either notice it because the are large and have an opinion on them or you notice them because you like breasts and well... There are two very large breasts there. If it wasn't like this the the question wouldn't have been asked in the first place as you wouldn't have noticed.
B) Art style. Most overly exaggerated bosoms are because of Asian influences on drawing the female form. Anime for example is a major contributor for focusing on a woman's sexuality and form by having large breasts. **See also focusing on upskirt angles to show they are female**
C) Sometime it is just a fetish thing, but for most games out there with age rating aim for people 16+ then its points A and probably B if it is a Japanese or American company. If the age rating it 18+ and basically just about sex well then yeah its just a fetish.
Marketing wise this isn't the 80s. Boobs are for everyone in he eyes of making people buy stuff and in 2018 you are allowed to enjoy any part of a male or female body regardless of you actual sexual orientation.
Final point. Sometimes women just have naturally massive breasts. Its just as normal as having very small breasts.
As an example of the evolution of the sexualisation of women in games just look at Lara Croft. 90s Lara was about attracting a male audience. 00s Lara was about making her more of an action hero for men and women then the films came along and cast Angelina boobs. Ironically the aim was to attract non-gamers to play the game. Essentially focusing in meat heads, the kind in the comments on here that say dumb stuff like gamers don't get laid...
Now Lara is Zbrush sculpted, scan of a human body, PBR textured character with aim to immerse the player in a more realistic environment which is more relatable for men and women without focusing on her equipment of generation.
So really that's why they make some character have massive breasts.
Most Helpful Girls
Why do they make main male characters in video games have fit toned bodies. Masters at fighting or agility or gunplay... Or be super charming.
That's also the sexualization of men to a certain degree. Simple. People don't want to play as a plain average looking or fat person. Maybe they would in an rpg. But as people said above sex sells. Maybe not to everyone. But it sells to the majority. I much rather have Lara Croft be a bad ass.. good looking.. Smart.. Agile character than a boring... weak one.
Doesn't mean I'm shallow. It rather means I like playing as a cool character. You get games with the characters I described and you get games where you can make whoever you please. Big boobs or small etc. Theirs games that cator to everyone's tastes. So dont let it bother you too much.
Those hands tho! Forgot the breasts those hands are scary! If the boobs was intended to make her hot i think guys will have nightmares about her crushing their junk for sure