Why different collar widths?
Wider the collars are more comfortable on the dog's neck because they reduce pressure. A wider collar will provide more support which makes them ideal for stronger dogs.
Do take your dog's breed into account. For example, if you have a small breed dog, a wide collar may not be suitable.
If you are unsure of which width to go for, opt for a 2.5 cm.
As a general rule:
Small breed dogs (adult weight up to 10 kg) ideal width is up to 2.5 cm
Medium breed dogs (adult weight up to 25 kg) ideal widths are 2.5 cm, 3 cm, and at a push 4cm. 4cm collars are usually only for pit bulls as they have a stronger pull.)
Large breed dogs (adult weight up to 40 kg) ideal widths are 3 cm or 4 cm.
Giant breed dogs (adult weight over 40 kg) ideal with is 4 cm.