Compare size to known stock boot images for your device. If it's wildly different (e.g., 200MB instead of 32MB), you likely included extra partition data.
Once stripped, repack using AIK or mkbootimg as needed.
Note for Xiaomi: Some devices have boot images that start at offset 0x00000800. In that case:
dd if=boot.emmc.win of=boot.img bs=1 skip=2048
If you renamed it but Fastboot says error: cannot load image, ensure your drivers are updated. If the file is verified as correct, try flashing it via TWRP (Install > Install Image > Select boot.img > Select "Boot" partition).