When opening two separate Muse sites I have received the warning message, 'This document contains a link to an asset being upscaled. You should resize it smaller, or relink to a larger asset using the Assets panel'. However, none of the assets that the site links to have been amended.
Does anyone know why this might be?
Ideally this never happens. Maybe be you modified the asset outside of Muse. What I'd suggest you is to resize the asset as suggested in the warning message and try to replicate the issue. I believe you won't get the warning again.
I believe the warning you're referring to is "This document contains a link to an asset being upsampled."
Upsampling refers to the fact the image has been resized to a size larger than it's original pixel dimensions. For example a 72x72 pixel square image may have been placed then resized to 100x100. Such an image will be exported by Muse, but it will not have an optimal appearance in the browser due to being larger than it's original dimensions. (It's been "upsampled.")
In general you want to have all the images in your site be the same size or smaller than the original placed/imported image data in order to avoid upsampling. Slight upsampling can result in an image that looks dull or slightly blurred, depending on the browser being used. A large amount of upsampling will result in an image that appears pixelated in the browser.
This message is a warning. Whether or not the result of the upsampling is acceptable is a judgement call only you can make.
The Assets panel displays an icon next to any image that's being upsampled.
One other case where this will be reported is for any image that is larger the 2048 pixels in width or height. Muse initially limits image data to a 2048 maximum in either dimension. If you have an image that is larger than 2048, you need to select it in the Assets panel, right click and choose "Import Larger Size" from the context menu. This will pull in the additional image data for whatever size the image is in Muse and thus avoid any upsampling.