I don't think you can go by the size on two seperate occasions I have had it through dpd, One was a hard drive and the other was a OC mat.
I think as she says it's a matrix, probably size/weight/value and possibly "which service has capacity"/"which service costs more".
I've had HDD's delivered by both DPD and RM from OCUK, I suspect for a bunch of things with the way RM have been upping their prices and the volume discounts OCUK are probably getting from DPD these days that it may not cost much more to send DPD.
If it was a big OCUK mat it would possibly have been cheaper to have sent via DPD than RM due to the size when rolled up (if it's over 45cm on any side it becomes a large parcel or possibly tube, whilst DPD probably still go by weight).
Something I realised years ago was that sometimes as a consumer a courier/Parcel Force was cheaper for the same size item than RM.