If Thunar is set to detailed view and I use STRG and turn my mousewheel nothing happens to the Files icons. If Thunars View is set to Symbols or Compact the zoom on Icons and Previews works like it should. I tested Thunar 1.6.15 from a Live-System and it also zoomed in detailed view.
Thanks for reporting! But sorry, you are too late, already fixed. :) Fix will be released in thunar 1.8.12 *** This bug has been marked as a duplicate of bug 15936 ***