Mac app shrinks when copied to USB
Your thumb drive is likely formatted as FAT32 (or just FAT if older).

Copying the app to something that isn't formatted as Mac OS Extended, probably resulted in the loss of things like file owner, permissions, etc, that may have been needed to run/validate it with the system.

Either consider


Packaging it as a .dmg, or
If this is something you might need often, you could reformat the thumb drive as HFS+ (after backing up anything important on it).  Consider copying the app file with the flag to preserve settings:

cp -p source_file target_thumb_drive