Changing the crank arms probably just means changing the length. If you go from a 170mm arm to a 172.5 your pedal at the bottom of the stroke will be closer to the ground. You will then have to lower your saddle so that you maintain the proper knee angle. Shortening your crank means the opposite. Again, you sacrifice optimum position. Choice of crank length is made on leg length, cadence preference, etc. I wouldn't make it based on amount of cleavage.