PCB Library Expert PTH Reference Calculator

The Hole Size is calculated over the maximum Terminal Lead and the pad stack is 1.5 X the Hole Size. 

A 0.50 mm Hole = 0.75 mm pad size or 0.125 mm annular ring, but the minimum annular ring overrides that. If your minimum annular ring is 0.20 mm then the pad size would be 0.25 mm

A 1.00 mm Hole = 1.50 mm pad size or a 0.25 mm annular ring. 

A 2.00 mm hole = 3.00 mm pad size or a 0.50 mm annular ring. 

Terminal Leads that create large hole sizes probably carry a lot of current or the package is big and bulky and has weight to support. That is the main reason for the larger Annular Ring. To support the package weight and voltage current. Requires additional solder.