Partial specific volume and compressibility properties of the extended state of proteins are estimated from additivity schemes using revised amino acid and peptide data. These calculated properties are compared with the experimental data of the native state in order to assess the contribution from folding. Results of this treatment show that, in the case of partial specific volumes, there is close agreement between the two data sets for a number of proteins. The implication is that subtle compensatory contributions in volume occur during the folding process. In the case of compressibilities, however, a substantial difference is observed which is believed to arise because of the hydrophobic interior created in the native protein as a result of the folding process. Using suitable measures of protein hydrophobicities and estimates of the fraction of buried apolar residues, a "micellar model of protein compressibility" is proposed and tested for several proteins. Results obtained from this model show good agreement with the experimental data for the native state of a number of proteins.