Are you still supposed to tip 20-30% on HIGH bills?

European here with a mild fascination for US tipping culture

The percentages that people post here really blow my mind. Sure, a 30% tip on a single beer when you send an hour in a pleasant bar.. I get that. 30% on a 100usd meal that was really good with exceptional service.. sure..

How does it work when you order, say 8 pizza's for 120usd total? Do you give 5usd since its the same effort for the delivery guy? Or are you expected to give 20usd because otherwise 'the percentage' is an insult?

What if you buy dinner for a large group of 10 people and the total bill is 1k.. are you really supposed to add 200-300usd? For the restaurant it's pbb not much difference in effort to serve 5x2 people or 1x 10.. win some on taking orders, lose some on having to serve 10 people at the same time.. 5x2 people would equate to that 200-300 tip..