How to resolve this issue:

Web Designer 5 / Xara Xtreme 5

Right click on each large photo you have in your document and select "Optimize Photo". This reduces the size of the image whilst keeping the quality high.

Web Designer 6 / Designer Pro 6 and later

Go to Utilities | Optimize all JPEGs. This will scan the document for any photos that have not been optimized already and optimize them.

Please view the following article for information on photo optimization which will reduce your file size and amount of memory required during export.