I'm doing a comparative study between ATtiny84 and ATtiny85, and I found from the datasheet that ATtiny84 consumes less current, is that normal even if it is bigger in terms of size and number of pins? Am i wrong?
There are wide differences in power usage in AVR parts. Typically the "P" variants (for "picopower") are more optimized for low power usage, but there are also differences between different revs of the same part (for example the ATMEGA168PB uses slightly more power than the ATMEGA168PA during sleep) presumably becuase of different processes used to make the chips.
If you are looking for the lowest power possible, it will depend on how your application. Do you spend most time running at full speed? Power down? Do you need a realtime-clock?
Don't forget about the ATXMEGA parts- they have some that have very low power when running a real-time clock from the internal "ultra-low power" oscillator.