A large party requires more co-ordination in getting the appetizers out at the same time, the main course, the desserts - some of that is the chef’s responsibility of course. Large parties tend to sit longer - a single diner might be out in an hour, a couple in 90 minutes, a large group in two hours - fewer sittings, fewer tips.
If you have six small tables and one of them stiffs you and five tip, that’s not so bad. If you have a large table and they stiff you, that really hurts, because you’re getting paid $2 an hour by the restaurant. The IRS assumes you got tipped 7% of your checks - a table that ran up a check of $200 stiffs you, the IRS taxes you on $14.
So, are you saying that the IRS knows how much the lunch tab was for every table you served? Somehow, I sincerely doubt that.
The IRS will tax you on your W-2 earnings, and your REPORTED tips. And we are all well aware that every server always reports every dime of tip they get a night, right?