Voltage divider with lower ohm rating
These resistors are used simply to provide very specific voltages in order to notify the connected iDevice that it is able to draw a lot of current for charging.  The voltage you'll get will still be the same as long as all of your resistors are off by the same factor, so you can get away with your resistors off by a few orders of magnitude.

The reason such high values are used is because resistors waste power.  The data line connections D- and D+ are high-impedance, meaning they require very little current, and high resistor values are acceptable.  The original 43K, 51K, and 75K resistors will waste around 93μA (plus a tiny bit of current into the data lines), which is quite small.  Using 43, 51, and 75 ohm resistors will waste around 93mA, which starts to be significant.  To make matters worse, that wasted power has to go somewhere, and physically small resistors can have trouble dissipating it.

You don't want to increase your resistor values too high either, because the current leaked into (or out of) the D+ and D- lines will start to become significant, and will throw off your desired voltage.  In the end, deviating in resistor values by a few orders of magnitude in this case won't change much, but there is definitely a good reason for the specific values given.