Power in a digital CMOS chip is proportional to clock frequency, how many gates are clocking(*), and supply
voltage squared. (plus some losses from the crystal oscillator and analog circuitry in practice)
Thus the 3.3V 8MHz ATmega running normally consumes less than a quarter of the power as at
16MHz and 5V...
(*) actually the total capacitance of all signals / gates that are clocking, thus f C V^2
By the way trying to get the current consumption below a few uA may be fairly pointless - the leakage
current in your battery will start to dominate at some point, so you may be making no further
improvements to battery life. 5 years at 5uA is 220mAh of charge (pretty small battery), yet some
battery technologies self-discharge on faster timescales than this.