A method of increasing the dosing accuracy of an injection device of pen-injector type, the method comprises the steps of: - providing the injection device for delivery of a liquid medicament having a housing and a drive mechanism with a piston rod to advance in a distal direction against a proximally facing surface of a piston of a cartridge, and further having a dose setting member and a dose dispensing member to set and to dispense a dose of the medicament, wherein the drive mechanism further comprises a dose indicating sleeve rotatably supported in the housing and comprising a helically-shaped dose indicating scale (82, 84) on an outer circumference of the dose indicating sleeve to appear in a window of the housing to indicate setting and dispensing of a size of the dose of the medicament to a user in accordance with a dispensing rate of the medicament when the piston is displaced, and wherein the drive mechanism is configured to advance the piston rod in distal direction by a distance z1 to dispense a reference volume V1 of the medicament when operably engaged with a reference cartridge having a reference diameter D1, - arranging a cartridge inside the housing instead of the reference cartridge, wherein the cartridge has a tubular shaped barrel filled with a liquid medicament and is sealed in a proximal direction by a piston displaceably arranged in the barrel, wherein the cartridge has a diameter D2 smaller than the reference diameter D1, thereby enabling setting and dispensing of a dose of the medicament of a volume V2, smaller than the volume V1 when the piston rod is advanced in distal direction by the distance z1, wherein a ratio R of the diameter D2 and the reference diameter D1 according to R = (D2 / D1) 2 is smaller than 1 but larger than 0.1, and - resizing the dose indicating scale (82, 84) by the ratio R compared to a reference dose indicating scale applicable with the dose indicating member when the drive mechanism is operably engaged with the reference cartridge. - providing the injection device for delivery of a liquid medicament having a housing and a drive mechanism with a piston rod to advance in a distal direction against a proximally facing surface of a piston of a cartridge, and further having a dose setting member and a dose dispensing member to set and to dispense a dose of the medicament, wherein the drive mechanism further comprises a dose indicating sleeve rotatably supported in the housing and comprising a helically-shaped dose indicating scale (82, 84) on an outer circumference of the dose indicating sleeve to appear in a window of the housing to indicate setting and dispensing of a size of the dose of the medicament to a user in accordance with a dispensing rate of the medicament when the piston is displaced, and wherein the drive mechanism is configured to advance the piston rod in distal direction by a distance z1 to dispense a reference volume V1 of the medicament when operably engaged with a reference cartridge having a reference diameter D1, - arranging a cartridge inside the housing instead of the reference cartridge, wherein the cartridge has a tubular shaped barrel filled with a liquid medicament and is sealed in a proximal direction by a piston displaceably arranged in the barrel, wherein the cartridge has a diameter D2 smaller than the reference diameter D1, thereby enabling setting and dispensing of a dose of the medicament of a volume V2, smaller than the volume V1 when the piston rod is advanced in distal direction by the distance z1, wherein a ratio R of the diameter D2 and the reference diameter D1 according to R = (D2 / D1) 2 is smaller than 1 but larger than 0.1, and - resizing the dose indicating scale (82, 84) by the ratio R compared to a reference dose indicating scale applicable with the dose indicating member when the drive mechanism is operably engaged with the reference cartridge.