The issue is that you want all of the weight of your car to be useful weight, mass that serves a purpose. If your car is under weight or off balance, get something, ANYTHING that will help it go faster instead of just adding sticky weight strips. With all else being equal meaning two packs of the same internal resistance and voltage profile the one which is a bigger pack, more milliamp hours, will make your car more powerful over the run and therefore go faster. Also, as a rule with everything else being equal, a larger pack will have less internal resistance because it has more area to source electrons from. So, when you yank the trigger and put a big load on that battery coming out of the corner, your tiny pack drops from 8 volts to 7, where as the big pack drops from 8 to 7.5, just that HALF of a volt means 15% more power, and 15% will get you a car length or more over the course of the lap.
Think about it this way: Did ANYBODY in the A-main of the 17.5 nationals think to themselves 'I want a smaller battery (for any reason) and just make up the difference with lead'? Hell no, massive packs all around. Make your cars weight work for you instead of hold you back.