A rotor structure for a compressor, the rotor structure comprising: a tie bolt that extends along a rotor axis between a first end and a second end of the tie bolt; a first rotor shaft (104 1) fixed to the first end of the tie bolt; a second rotor shaft (104 2) fixed to the second end of the tie bolt; a plurality of impeller bodies (106 1 -106 n) disposed between the rotor shafts (104 1,104 2), the plurality of impeller bodies (106 1 -106 n) supported by the tie bolt and mechanically coupled to one another along the rotor axis by way of a plurality of hirth couplings (108 1 -108 n-1), wherein a radially-inner contour of an impeller body of the plurality of impeller bodies (106 1 -106 n) has at least two distinct axially-extending zones (Z1,Z2,Z3) each having a respective geometry configured to balance mass distribution about the rotor axis and control relative radial and/or axial growth between corresponding interface locations along the rotor axis at which corresponding faces of a respective hirth coupling of the plurality of hirth couplings (108 1 -108 n-1) mesh with one another, wherein the radially-inner contour of the impeller body comprises a first axially-extending zone, a second-axially-extending zone and an intermediate axially-extending zone, which is disposed between the first and second axially-extending zones (Z1,Z3), characterised in that the respective geometry of the intermediate zone is characterized by a varying bore diameter along the rotor axis, wherein a maximum of the varying bore diameter of intermediate zone is larger relative to the respective bore diameters (D1,D3) of the first and second axially-extending zones (Z1,Z3). a tie bolt that extends along a rotor axis between a first end and a second end of the tie bolt; a first rotor shaft (104 1) fixed to the first end of the tie bolt; a second rotor shaft (104 2) fixed to the second end of the tie bolt; a plurality of impeller bodies (106 1 -106 n) disposed between the rotor shafts (104 1,104 2), the plurality of impeller bodies (106 1 -106 n) supported by the tie bolt and mechanically coupled to one another along the rotor axis by way of a plurality of hirth couplings (108 1 -108 n-1), wherein a radially-inner contour of an impeller body of the plurality of impeller bodies (106 1 -106 n) has at least two distinct axially-extending zones (Z1,Z2,Z3) each having a respective geometry configured to balance mass distribution about the rotor axis and control relative radial and/or axial growth between corresponding interface locations along the rotor axis at which corresponding faces of a respective hirth coupling of the plurality of hirth couplings (108 1 -108 n-1) mesh with one another, wherein the radially-inner contour of the impeller body comprises a first axially-extending zone, a second-axially-extending zone and an intermediate axially-extending zone, which is disposed between the first and second axially-extending zones (Z1,Z3), characterised in that the respective geometry of the intermediate zone is characterized by a varying bore diameter along the rotor axis, wherein a maximum of the varying bore diameter of intermediate zone is larger relative to the respective bore diameters (D1,D3) of the first and second axially-extending zones (Z1,Z3).