"can hex changed to asm?"
Yes. With a disassembler. If the program isn't very small disassembling is a VERY difficult task and usually takes less time to rwrite the code by yourself.

"what is the different between the bin and hex,"
bin is the program data, hex is a file format so the data can be read and interpreted by eprom programmers and other devices

"why bin smaller than hex?is the bin is for differnet microcontroller?hex is for common 8051???"
bin is the data only. hex has headers, checksums. look at the SEARCH button and the look for hex format, there is a good documentation about this.