Szenario:

• ACP: Attachment settings
  • Maximum file size: 500 KiB
  • Maximum image dimensions: 1000 x 1000 px
• JavaScript in browser activated
• prosilver

Upload an image with the size: 1680 x 1050px and filesize: 483 KiB

As an admin:

• the image gets uploaded and resized to 1000 x 625 px
• image name clickable
• status: green checkmark

As an user:

• the image gets processed
• the image name is not clickable
• status: warning icon
• click on "warning icon" displays: The file is too big, maximum allowed size is 500 KiB.

The displayed warning message is wrong and misleading, the user will not get, why his image was denied.

Edit:
Different picture:
Upload an image with the size: 1680 x 1050px and filesize: 201 KiB.

Everything works fine. The file size is displayed with 201 KiB, but is actually 223 KiB. So the first file gets his file size increased from plupload up to 580 KiB. So the message is correct, but misleading, due to the reason, that the posting editor displays still the first (original file size), which is no problem.