What is the internal resistance of a LiPo battery determined by?
Internal resistance is dependent on a number of factors, including:-


electrode and separator thickness 
porosity of electrodes and separator 
active material particle size and type
electrolyte chemistry 
interconnects


A lower internal resistance is desirable for improving the maximum discharge rate, as it reduces internal voltage drop and heating. Most lipos use similar chemistry, but high power cells need thicker plates with larger interconnects to lower the metallic resistance and avoid 'hot spots'. This makes the cell larger and heavier than one  which is optimized for capacity.