The pKa plugin gives the following pKa values:
diethyl malonate: 13.0
diethyl methylmalonate: 14.7
diethyl ethylmalonate: 15.9
diethyl propylmalonate: 16.3
diethyl isobutylmalonate: 16.7
diethyl cyclohexylmethylmalonate: 16.9
I find it hard to believe that the pKa values of these compounds increase so dramatically with such small changes in substitution. Can you verify?