Unless you see the tang bullying him quite regularly i doubt that its the tang. We really need your water parameters though to get a good idea whats going on. please post
Any additional info you can provide us about your system would help to get us closer as well. How much live rock (in pounds) and how long the tank has been established. What do you use for filtration?
On the unlikely event that it is the tang this shouldn't really be an issue because you will really need to start finding a better home for your tang. Even though he is small, size isn't as much of the issues as room to swim. All tangs like to have lots of room since they are such active fish and will not be happy until they can stretch their fins so to speak. Not sure what the length of your tank is but for a yellow tang i would not suggest keeping it in less then 90 with 4' of length. 6' would probably be much better. You may be able to trade the tang in for credit at your LFS
and kill two birds with one stone. The fish will have a better home and it will eliminate the possibility that it is bullying your puffer.