If I consume $30 worth, and tip 10%, waiter gets $3.
If there’s two of us, and we consume $60, the waiter probably does 60% extra work for 100% extra tip at 10% of the total bill ($6)
I would have thought (but perhaps I’m wrong) that the waiter does less work per person as the number of people at a table increases, so why would the percentage tip go up?
And as far as the bill goes, if there’s only me, I’ll likely stick to soft drinks. Whereas when there are more people in my group, I’d be much more likely to grab a bottle of wine to share, pushing the total bill per person up, and thus the tip.