Players of indie porn games have, by and large, learned the language of those indie porn games. Big dick=protagonist. Small dick=opponent/cuck. Big tits=hot girl/milf. Medium to small tits=18-year-old stand-in for younger teen/sometimes an Asian lady. Seeing another guy bang any lady=cuckold simulator, sex with somebody with a penis and small boobies=heterosexual sex that shows up late in a large game's development cycle, minigames=chores that stand in the way of sexytimes, etc. etc.
When people mess with those equations, it gets difficult for some players to appreciate the game, and they start to worry they're getting bait-and-switched or that something will get in the way of their immediate gratification. It also doesn't help, re the original topic, that culture is so fixated on the idea that bigger=better up to the point that there's basically no upper limit on how much bigger a dick can be before it stops being better. In a way, I think this is the fallout of some of the more famous written porn (greyarchive, Fenoxo's stuff, Free Cities, lots of stuff on literotica) going beyond the realms of feasibility and even at times possibility when fantasizing about how big 'big' boobies actually are, and how big a 'big' dick is. Often, if we're honest, because they haven't had much or any real sex beyond masturbation and rely on Internet stereotypes for basic sexual education even about things like what cup size big boobies are. IRL, on a thin woman, C-cups are big boobies. IRL, 6 inches is a long dick. But in porn games, D-cups often mean a lady is the least stacked of the bangable girls (my God, what jailbait! She ONLY has D-cups! She's just there for the guys who wanna fuck high school girls or younger!), and 6 inches is ridiculously small for a penis in most porn games, to the point where girls might well point and laugh at a 6 inch dong in a porn game.
Now, I don't think any of this means devs can't or shouldn't make games with more realistic dick proportions. Lots of games have already caught on to the idea that B-cups can indeed be lovely boobs (for clarity, I'm on team C-cups or bigger please, but I don't begrudge people making games for other people too), so I don't see anything special preventing a general shrinking of dick sizes by some porn game devs as well. The more difficult to overcome barrier is probably just that it's uncommon in porn games, and thereby a risk.
But on the other hand, it's shitty to tell a game dev that they're wrong to put only big dicks in their game, or that it's bad to have a big dicked protagonist, or that they can't have huge titties in their game, or even that all the sex and proportions have to be not just realistic but relatively common. Indie porn games are great because people can, in theory, make whatever the fuck they want, even if they often fall into similar tropes to what others do. So, much in the way that there have been more gay games lately rather than fewer heterosexual games, I think there should be more average-dick-size (or smaller) protagonist games, but not fewer big dick protagonist games.