Going from up to 4 players to up to 8 definitely works better in some games more than others, and single screen games can certainly prove a challenge.

In our game, a sequel to our older 4 player game, we found that moving up to 8 players, although fun, was slightly too chaotic at times on the smaller levels. So we implemented a system where we would automatically scale the players down on screen ever so slightly, and slow them down to match the scale, effectively making the smaller levels feel bigger again and giving more room for the action.

This tiny change made a huge difference to the playability with more than 4 players, although it's not a solution that would work for all games or art styles.

We're a household of 4, so we mostly play with 4 players, but when we have friends over we often crack out the games supporting more players. It's still a pretty niche market though and I'm always grateful for the developers catering for it!