hii Liudr,i did some research & found out that mega 2560 cant be programmed through usbasp programmer.
there are some problems with the "fischel" firmware but cheap $2 chinese ebay versions do an excellent job. imo best programmer out there.
and correct usbasp firmware
but after that any program uploaded through the arduino as isp not working at all & in the process boot loader also get corrupted.
so in a nutshell my avrs are always set to boot area even for apps at 0.
im surprised its any different for arduino mega.