Sorry for chiming in late however i do agree you should aim at feeding something a bit more nutritional and filling... Also, if you havent dewormed your puffer, it maybe a good idea to do so unless you are sure that your puffer came in 100% healthy which more often than not is not the case...
On a final note, there was once another member here who mentioned that their puffer remained small for a very long time as well but IIRC there was not definitive answer as to why they might have been case, coz that member also gave his puffer a larger than necessary (at the time) tank and was feeding his puffer a well rounded diet + dewormed...