I'm thinking:-
1) Corrupt iconcache. Delete and re-boot. (C:\Users\Elmer B. Fuddled\AppData\Local). iconcache.db is, by default, a hidden file.
2) Corrupt thumbs cache. Delete through Disk Clean Up. As above, re-boot.
Note on the second option you may get the "green loading bar" behind/in the address bar as the cache is being rebuilt.
My money is mainly on option 2, as large icons are actually the compressed png image, not an ico image.
A third possibility (if third party) is that the max size contained in the ico file is 48 x 48 pixels.