除非我弄错了，否则图像大小不应该像绘图区域那样改变：

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([1, 2, 3, 4])

print('Before tight_layout')
img_width, img_height = fig.get_size_inches()
bbox = ax.get_window_extent().transformed(fig.dpi_scale_trans.inverted())
print(f"Fig size:  ({img_width:.2f}, {img_height:.2f})")
print(f"Axes size: ({bbox.width:.2f}, {bbox.height:.2f})")

fig.set_layout_engine('tight')
fig.savefig('image.png')

print('After tight_layout')
img_width, img_height = fig.get_size_inches()
bbox = ax.get_window_extent().transformed(fig.dpi_scale_trans.inverted())
print(f"Fig size:  ({img_width:.2f}, {img_height:.2f})")
print(f"Axes size: ({bbox.width:.2f}, {bbox.height:.2f})")

plt.close()

输出：

Before tight_layout
Fig size:  (6.40, 4.80)
Axes size: (4.96, 3.70)
After tight_layout
Fig size:  (6.40, 4.80)
Axes size: (5.78, 4.26)

检查：

arr_height, arr_width, _ = plt.imread('image.png').shape
print(f"Image size: ({arr_width / fig.dpi:.2f}, {arr_height / fig.dpi:.2f})")

# Output
Image size: (6.40, 4.80)