In roasting we usually underload our full drum capacity in order to create specific roast profiles. Smaller batch sizes will potentially give you a much higher rate of rise (RoR) - this is the temperature increase during the roast process, usually measured in 20 or 30 seconds.
Typcially we roast espressos with batch sizes of 70-100% of total drum capacity, as we are looking for more body and intrinsic heat to be passed on. Filters are roasted with 40-70% because we are looking for racier profiles with shorter roast time and higher RoR so we can work on more subtle flavours, acidity and clarity.
Our old Probat UG15 roaster had a 15kg drum