You may have seen a “CR2032” cmos battery. The “2032” is broken out into “20” and “32” and means 20mm diameter and 3.2mm thick.  Similarly, a “2025” is 20mm diameter and 2.5mm thick.  So a 2025 is thinner than a 2032.  In the same way, a “1220” is 12mm diameter and 2.0mm thick, so smaller and thinner than both .

A “CR” means primary cell; an “ML” means secondary cell.