That makes sense for mail and goods.The actual capacity is set to this base capacity divided by 1 for mail, 2 for goods and 4 for all other cargoes. To override this effect, use the passenger_capacity and mail_capacity callbacks.
Mail is mass produced by towns so aircrafts are refitted to mail quite often.
1/2 goods capacity is fine since their transportation price is about twice as passengers.
That does not make sense for valuables, though.
They are produced and accepted by banks/towns making them very convenient to be transported between airports two ways. However, 1/4 aircraft valuables capacity makes them completely inferior comparing to passengers/mail transportation, even in abundance of valuables at the airports. Their transportation is not that lucrative to cover that low capacity.
Would it be better to set their aircraft refit capacity divider to 1 or 2 instead of 4? Thank you.