The shipped phpMyAdmin favicon is 18kB large:
-rw-r--r-- 1 thijs root 18902 2007-06-15 19:27 favicon.ico
That seems quite overkill for a 16x16 image, it seems to contain some extraneous data. I've recreated the same icon, and now it's just 1400 bytes:
-rw-r--r-- 1 thijs root 1406 2007-07-09 15:39 favicon.ico
Please find attached the updated file.