Who says size doen't matter?
The belly containers are much lower profile and many have a smaller foot print which restricts the size of what one carries. The A310 can carry a lot more in size and weight. In a previous life I worked for a LARGE operator of the type in the cargo configuration, along with it's larger sibling. We had spanning loads and special configurations that would not work in a lower hold. Also, there are various goods which cannot be carried aboard passenger aircraft.
I hope this answers your questions.