There is no comparison as they are different forms of measurement. A millimeter is a unit of length equivalent to 1/1,000th of a meter, whereas a liter is a measurement of volume equal to 1/1,000th of a cubic meter.
A cubic meter equals 1,000 x 1,000 x 1,000 millimeters, or 1,000,000,000 cubic millimeters. There are 1,000 liters in a cubic meter. Because 1,000,000,000 / 1,000 = 1,000,000, there are 1 million cubic millimeters in 1 liter. Typically, this volume is expressed in cubic centimeters because the centimeter is the most commonly used portion of a meter. A cubic centimeter equals 10 x 10 x 10 millimeters, or 1,000 cubic millimeters. Therefore, 1 liter equals 1,000 cubic centimeters.