I recall reading in the past that USBtinyISP had some problems in uploading to ATMega328p's due to the ATMega328p's having more memory than USBtinyISP was prepared to deal with. I did a quick google search and found: Desert Home: Arduino Mega2560 - Wrapping up the Bootloader Problem.
Nick Gammon also had this to say: http://www.gammon.com.au/forum/bbshowpost.php?bbsubject_id=11357.