I am working on atmega328 but i want to know which are the other atmega chips that are exactly the same but they are smaller in size....
Compiled code size doesn't tell the whole story; there's also the number of variables (and their types) you use, and a few other details (most minor) that may be relevant.
That said, the other two most direct options would be the ATMega8 and the ATMega168 - which have 8K and 16K of program space available (6K and 14K w/ standard bootloader, respectively). They also each have smaller SRAM sizes than the ATMega328, which is where the question of "how many and what type of variables are you using?" comes into play. Look at the spec sheet for more details.
Finally - while you can still find them, I think production has shifted to predominently the 328, so keep that in mind as well. Really $6.00 each (provided it is pre-bootloaded) isn't that bad, unless you are talking a major production run (in which case the number can be brought down some by buying thousands, and no bootloader - even so, it is still an expensive chip for a production-run product).
