iteration planning vs team size

For an Agile team of the ideal size of 6 people (team of real, rather than ideal, people) an efficient iteration planning process should take about 3 cycles to develop.

For a team of 15 it took about 8 iterations to make iteration planning efficient and relatively low in ceremony.
Iteration planning with a team of 30+ people failed miserably, and was abandoned after a couple of attempts.
A tiny team of one to two people arrives to efficient planning process by the second or third cycle, or not at all.

Those are the data points I witnessed first hand.
What's your experience?