This penalty applies to actions made while carrying the item other than attacking with it.
It’s a measure of how hard it is to do other things while carrying the item, whereas awkward is about if its easy to attack with.
Cumbersome is usually a function of length, weight, or size.
The penalty is not strict, but is applied using the story teller’s judgement.
For example: Cumbersome one from carrying a rifle might not apply to running on flat ground for example, but would almost certainly apply to scaling a fence. Trying to carry three rifles on the other hand would penalize running for sure.