13' ones use U (ultra-low power) processors, while 15' use QH (quad-core with high performance graphics). Same with Dell updating XPS 13 with Kaby Lake last month, but XPS 15 would have to wait till next year.

http://www.everymac.com/systems/by_processor/intel-core-i7-m...

https://superuser.com/questions/940168/meaning-of-intel-proc...