LEAN?
From their detailed spec:
http://www.iab.com/wp-content/uploads/2015/11/IAB_Display_Mobile_Creative_Guidelines_HTML5_2015.pdf
" For this release, a maximum of 15 file requests for initial file load is imposed to ensure little adverse effect on page load performance.
This file request limit only applies for the file requests required to display the ad upon initial load.
No file request limit has been placed on subsequent file loads."
15 & subsequently potentially lots more requests is not lean
As for the max (zipped) file sizes allowed - most people would regard them as way too big - their examples often a few hundred K each would still burn through a mobile contract low bandwidth limits rapidly .
Ad industry, still in their own bubble without a clue about the performance & bandwidth grief their unwanted ads cause.
Biting the hand that feeds IT
