I think the best measure of real-world battery improvement would be a chart of iPhone / Macbook battery densities over time. My guess is that there's been little improvement over the last 10 years.
iPhone 5: Dimensions: 51 x 39 x 3.6 mm (2.01 x 1.54 x 0.14 in) Capacity: 1440 mAh Voltage: 3.8 V
iPhone 5s: Dimensions: 51 x 39 x 3.8 mm (2.01 x 1.54 x 0.15 in) Capacity: 1560 mAh Voltage: 3.8 V
iPhone SE (1st generation): Dimensions: 51 x 39 x 3.95 mm (2.01 x 1.54 x 0.16 in) Capacity: 1624 mAh Voltage: 3.82 V
iPhone 6: Dimensions: 66.4 x 51.8 x 3.8 mm (2.61 x 2.04 x 0.15 in) Capacity: 1810 mAh Voltage: 3.82 V
iPhone 6s: Dimensions: 65.6 x 51.7 x 3.8 mm (2.58 x 2.04 x 0.15 in) Capacity: 1715 mAh Voltage: 3.82 V
iPhone 7: Dimensions: 73.1 x 32.4 x 5.2 mm (2.88 x 1.28 x 0.20 in) Capacity: 1960 mAh Voltage: 3.8 V
iPhone 8: Dimensions: 74.5 x 26.6 x 6.9 mm (2.94 x 1.05 x 0.27 in) Capacity: 1821 mAh Voltage: 3.82 V
iPhone SE (2nd generation): Dimensions: 67.3 x 40.9 x 3.73 mm (2.65 x 1.61 x 0.15 in) Capacity: 1821 mAh Voltage: 3.82 V
iPhone X: Dimensions: 88.9 x 32.5 x 3.25 mm (3.5 x 1.28 x 0.13 in) Capacity: 2716 mAh Voltage: 3.81 V
iPhone XS: Dimensions: 87.16 x 32.61 x 2.96 mm (3.43 x 1.28 x 0.12 in) Capacity: 2658 mAh Voltage: 3.81 V
iPhone XR: Dimensions: 76.0 x 30.8 x 3.25 mm (2.99 x 1.21 x 0.13 in) Capacity: 2942 mAh Voltage: 3.81 V
iPhone 11: Dimensions: 90.0 x 31.9 x 3.25 mm (3.54 x 1.26 x 0.13 in) Capacity: 3110 mAh Voltage: 3.83 V
iPhone 12: Dimensions: 81.5 x 32.4 x 4.1 mm (3.21 x 1.28 x 0.16 in) Capacity: 2815 mAh Voltage: 3.83 V
iPhone 13: Dimensions: 84.5 x 30.9 x 5.7 mm (3.33 x 1.22 x 0.22 in) Capacity: 3095 mAh Voltage: 3.83 V