Hi. The Image Size value almost never is the same as the size of the final file even with PSD files because it's affected by multiple factors like layers used, options selected when you save it, etc.
From the official help: "Another factor that affects file size is file format. Because of the varying compression methods used by GIF, JPEG, PNG, and TIFF file formats, file sizes can vary considerably for the same pixel dimensions. Similarly, color bit-depth and the number of layers and channels in an image affect file size."
Source: https://helpx.adobe.com/photoshop/using/image-size-resolution.html
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