Why do you need more labeling product for a cDNA array than for a BAC array?
The reason for using more genomic DNA for the cDNA array is because the BAC arrays have much longer DNA fragments on the array than cDNA, therefore higher homology to the labeled DNA than the cDNA array. The fluorescence signals from the cDNA will be weaker than the BAC array, therefore, you have to start with higher amounts of starting materials.