Thank you, it makes more sense when we know what R1 and R2 are!
The reason why higher values are suggested in that schematic is simply so that less current is wasted. Some output current flows directly through the resistors to ground so if you use unreasonably low values you sink the current for no reason and just generate heat in them. However, if you use values too high, you will restrict the base current in Q1 and it will no longer stabilize the voltage. The values are not critical (although their ratios are!) and ideally should be chosen so they are low enough to allow Q1 sufficient base current, with a safety margin, but not much bigger.
Brian.