Ok let me share some experience.. previously i encounter voltage drop problem, but now has been solved..
normally if our car is stock say NS60 and you upgrade to NS70.. that's just minimal upgrade or i should say not much different.. but if you are upgrading from NS60 to NS90.. den the problem you have is only the recharging back of the battery will be slow.. it's just like imagine you have a water tank in your house.. where the water tank is 100 litres and the incoming water supply is about say 30litre/min.. and your outgoing supply is constantly 50litre/min.. you might find the water tank i just barely enough to support as when it started to draw the water(current) you will be able to see the water incoming supply will keep turning on to support the water tank because of it's water loss.. so if you upgrade to a higher capacity this will be able to help more or less as most of the batteries/water tank will have a minimum water/voltage level to maintain till restart the charging...
so in this case if you change to a battery that is way too big, and your drainage is also high, your alternator will become vy stress.. it'll charge continuously.. as minimum charging will kick in @ 12.3V.. so whenever your car battery voltage goes lower than 12.3V your alternator will start charging again.. So conclusion is, you can change to big size but not too big like NS100 or so..