Larger parties do tend to undertip - one party of 6 can often result in lower tips than 3 tables of 2. If they split the check, invariably one party will undertip though others tip standard. I think they assume that others are covering so they can skimp a bit. Or if one person covers the full bill, the dollars can get high so they tend to go lower.

This was my experience being a server long ago. My family owned a restaurant and it was common knowledge that large tables meant worse tips. This was so long ago we never did automatic gratuities but servers typically would not want to take big groups. Sunday lunch crowd was the worst in terms of tipping. Back then cash was common too and sometimes one person would collect then pay and they would skim the tips. You'd watch them collect money and everyone was putting in tips, then they would lower their own cash contribution by using other tips to cover their bill or leave like 5% if using a card. Even in the 1990's before tip inflation, 5% was not a good tip. This was most common with co-worker lunches; we knew they were doing it too because if same people came in without this person, the tips would be standard. But there's nothing we could do as the server.

Separately, I do think that tourists undertip by American standards but that's just because our tipping culture is wild. I do tend to see more signs at US tourist areas which say "gratuity not included" or it's printed on menu.