Running wtperf with the following configuration will generate maximum page sizes that are aprx 7x larger than the given memory_page_max size.

conn_config="cache_size=100MB,statistics=(all),statistics_log=(wait=10)"
database_count=1
table_config="leaf_page_max=4k,internal_page_max=16k,leaf_item_max=1433,internal_item_max=3100,type=file,memory_page_max=10m,split_pct=90,leaf_value_max=1MB"
icount=50000000
populate_threads=10
random_range=10000000
report_interval=5
run_time=0
threads=((count=1,reads=1),(count=1,inserts=1))
value_sz=100
warmup=10

This run takes 30~ seconds to form the first very large page when running on my OSX Macbook Pro

Relates to MongoDB ticket SERVER-16902