I was of the impression packages over 1 kg were cubed - and only if they were of 'boxy' dimensions - i.e. - if you can pack it in a bag the shape of a ball - it cannot be cubed.

AP has a clause about a "notional cubic weight" where they take the longest, widest, highest dimension of the odd shape to make the cube.

AP have done it to me rarely ( I bung the package in a expandle bag if they do and then they can only weigh it) 

generally my local PO just do it by weight... I worry sometimes that an officious postal worker at the other end will notionally cube meaning I or the customer will end up with an underpaid bill