The use of He 111 in the Stalingrad airlift, as well as several other non-transport types, was problematic at best. Aside from the amount of load that could be carried, there are issues of loading and unloading. The 111 had no cargo doors so the size of what could be carried was limited. Loading and unloading also was much slower.
On the whole, these aircraft could only deliver a fraction of what proper transports could. But, since the Germans had no alternatives they were pressed into service.
As an added negative, by pressing these bombers into service they could not be employed as bombers in any attempt to aid ground forces trying to relieve the encirclement. So, it really amounts to a double negative.