@mietze Actually now that I thought about it, some issue are typically trivial to address in smaller group but increasingly tricky to tackle as we add people to them:
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Sharing a vision of what the game is about and what PC concepts look like; with four players you can chat and come to an agreement this is a low magic setting playing a rugtag group of peasants in situations way over their heads. As you add more folks you'll also start getting pitched more edge cases - noblewomen in disguise, excommunicated paladins, former assassins, etc.
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Not having enough TLC to go around by the GM. With a party of four more or less everyone gets to shine. As the ratio of GMs to players worsens so does the chance someone is left sitting on their thumbs.
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Administrative issues and logistics increase with the playerbase's size. There's a higher risk someone won't get along, you need to track down more stuff than just +sheets, juggling timezones and availabilities gets tricky, and as you add more GMs (well, staff) they need to be managed to like everything else.
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Continuity in general is something MU* don't handle very well because staff are so transient. A typical table-top campaign rarely survives its GM's departure but staff gets replaced on games all the time, leaving storylines dangling and character arcs unresolved. Even the style of play can be quite different based on who's running a plot which isn't something more intimate campaigns (often ran and involving well-knit groups who know each other well) have to struggle with.