Why isn't the yield for 1 µm scale syntheses five times higher than 0.2 µm scale syntheses?

For the 0.2 µmol synthesis scale, monomer coupling is done at a 40-50-fold excess. Doing so for larger scale synthesis (such as for a 1.0 µmol scale) would be cost-prohibitive. Large-scale syntheses are done only at 10-fold mole excess of amidites. To increase the final yields of these large-scale synthesis, the coupling times are extended, so to increase coupling efficiencies.