image inspect reported size is different from docker

### Description

Title says all

### Steps to reproduce the issue

docker image inspect vs . nerdctl image inspect

### Describe the results you received and expected

Reported size are different.

### What version of nerdctl are you using?

1.7.6

### Are you using a variant of nerdctl? (e.g., Rancher Desktop)

None

### Host information

_No response_