I don't think that the size of the resultant fire is anything other than what would have been expected. The Il-76 has a range of 3,650 km (1,970 nmi) at max payload (
Ilyushin Il-76 - Wikipedia, the free encyclopedia), whereas the straight line distance between Karachi and Khartoum is 3,721 km (2,009 nmi) (
ASN Aircraft accident Ilyuhsin 76TD 4L-GNI ? Karachi-Jinnah International Airport (KHI)). Max payload is about 45 tonnes, with the aircraft reported as carrying 36-40 tons of cargo. So the fuel load at departure is likely to have been approaching maximum.