Sounds like you’ve never waited tables!
Two-tops are the best for earnings, both for the restaurant and the server. Easier for a charismatic career server to separately charm three couples on dates than a single table of six all at once. And higher turnover is better for revenue. 1-on-1 business meeting? They’ve got work to do. Couples on dates? They’ve got fucking to do. Smaller parties -> faster turnover -> more money.
Large parties are worse in every way. Special table arrangements for reservations made in advance mean more seats left empty while waiting for the arrival. People at large gatherings will buy less food/drink just as often as they’ll buy more. Sometimes it’s a work party where everything is expensed; cool! But lots of companies don’t cover alcohol expenses, which is your most profitable item. Just as often it’s a going away party for a bunch of retail workers where everyone nurses one drink for two hours and splits 3 appies across the whole table.
Large parties suck. Some servers prefer them, but those people are heroes (and rare). Minimum 18% for large parties is standard where I am, but if it was my call it would be 30.